Method for air-fuel ratio control of a lean burn engine

ABSTRACT

A method is disclosed for controlling operation of an engine coupled to an exhaust treatment catalyst. Under predetermined conditions, the method operates an engine with a first group of cylinders combusting a lean air/fuel mixture and a second group of cylinders pumping air only (i.e., without fuel injection). In addition, the engine control method also provides the following features in combination with the above-described split air/lean mode: idle speed control, sensor diagnostics, air/fuel ratio control, adaptive learning, fuel vapor purging, catalyst temperature estimation, default operation, and exhaust gas and emission control device temperature control. In addition, the engine control method also changes to combusting in all cylinders under preselected operating conditions such as fuel vapor purging, manifold vacuum control, and purging of stored oxidants in an emission control device.

BACKGROUND OF INVENTION

1. Field of the Invention

The field of the invention relates generally to variable displacementengines for providing improved fuel economy.

2. Background of the Invention

Various methods are available for air-fuel ratio control. In somesystems, as the driver adjusts the throttle, fuel injection is adjustedto maintain a desired air-fuel ratio. In other systems, airflow isadjusted via an electronic throttle to match injected fuel.

The inventors have recognized a problem. In particular, if fuel iscontrolled to control engine output, then during the stoichiometric modefuel cannot be used to control air-fuel ratio since it is constrained tocontrol engine output. Alternatively, if fuel is controlled to controlthe air-fuel ratio, then during lean operation fuel cannot be used tocontrol engine output since it is constrained by air-fuel ratio.

Also, the inventors herein have proposed operating some cylinders withsubstantially no injected fuel, while operating other cylinders atvarious air-fuel ratios. This increases the lean combustion limit (sincethe engine is operating at a higher load) and reduces engine pumpingwork. However, if an exhaust air-fuel ratio sensor sees a mixture air(from the cylinders without injected fuel) and combusted gasses (fromoperating cylinders), then the sensor reading does not match thecombustion air-fuel ratio.

SUMMARY OF INVENTION

Various of the above problems are solved by controlling an engine havingfirst and second groups of combustion chambers and an engine exhaust,operating the engine in a first operating mode where (i) air andsubstantially no injected fuel are provided to the first combustionchamber group, (ii) both air and injected fuel are provided to thesecond combustion chamber group at a lean air-fuel ratio, and (iii)injected fuel is adjusted to the second combustion chamber group. Theengine can also be operated in a second mode where both air and injectedfuel are provided to both the first and second combustion chamber groupsand at least one of the air and the injected fuel provided to both thefirst and second combustion groups is adjusted to control the air-fuelratio of the first and second groups.

In particular, torque is controlled with fuel when some cylindersoperate with substantially no injected fuel, while air-fuel ratio can becontrolled by, for example, airflow. In other operating modes, air-fuelratio can be controlled by injecting fuel to all of the cylinders.

In another aspect of the present invention, the method comprises readinga sensor output of a sensor exposed to a mixture of air andsubstantially no injected fuel in a first stream from the engine and acombusted lean mixture of air and fuel in a second stream from theengine, and adjusting a measured air-fuel ratio of the combusted leanair and fuel mixture to account for the air in the first stream. Themethod further comprises treating the combusted air and fuel in acatalyst coupled to the engine that retains oxidants including NO_(X)when operating lean and reduces the retained oxidants when operatingrich.

Here, the engine control corrects for the pure airflow that wouldotherwise cause the air-fuel sensor to give an incorrect indication ofthe combustion air-fuel ratio. Note, in the specific case of a V-8engine where 4 cylinders operate with injected fuel and 4 cylindersoperate with substantially no injected fuel, the desired air/fuel wouldbe double to account for the pure air being seen by the sensor.Alternatively, the measured air-fuel from the sensor could be halved.

In still another aspect of the present invention, engine air-fuel ratiois estimated based on catalyst temperature. In particular, the methodcomprises operating the engine in a mode where air and injected fuel areprovided to the first combustion chamber group and air and substantiallyno injected fuel are provided to the second combustion chamber group.The method further comprises adjusting one of the air and the injectedfuel provided to the first combustion chamber group to adjust theair-fuel ratio of the first group to change between lean and rich,measuring the temperature of the emission control device and calculatingthe air-fuel ratio of the first cylinder group based on the temperature.

In particular, when the engine is lean, there are virtually noreductants to react exothermically across the catalyst with excessoxygen, and therefore catalyst temperature indicates a value that wouldbe expected. However, if the combusting cylinders operate slightly richof stoichiometry, the rich gasses (reductants) can react exothermicallyacross the catalyst with excess oxygen (from the cylinders that havesubstantially no injected fuel) and thereby raise the temperature of thecatalyst beyond that expected. As such, the method can indicate when theengine transitions between lean and rich, and therefore can give anestimate of engine air-fuel ratio.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show a partial engine view;

FIGS. 2A-2D show various schematic configurations according to thepresent invention;

FIGS. 2E-2H show various flow charts relating to fuel delivery andadaptive learning;

FIG. 3A shows a high level flow chart for determining and transitioningbetween engine operating modes;

FIG. 3B is a graph representing different engine operating modes atdifferent speed torque regions;

FIG. 3C shows a high level flow chart for scheduling air-fuel ratio;

FIGS. 3D(1)A-D illustrate various engine operating parameters whentransitioning from eight to four cylinder operation;

FIG. 3D(2) shows a high level flow chart for controlling engineoperation during cylinder transitions;

FIGS. 3D(3)A-D illustrate engine operating parameters when transitioningfrom four to eight cylinders;

FIG. 3E shows a high level flow chart for controlling enginetransitions;

FIG. 4A is a high level flow chart for controlling engine speeddepending on engine operating mode;

FIG. 4B is a high level flow chart describing vehicle cruise control;

FIG. 4C is a high level flow chart showing engine torque control;

FIG. 4D is a high level flow chart showing vehicle wheel tractioncontrol;

FIG. 5 is a high level flow chart for correcting an output of anair-fuel ratio sensor;

FIG. 6 is a high level flow chart for performing engine diagnostics;

FIG. 7 is a high level flow chart for indicating degradation of anengine sensor;

FIG. 8 is a high level flow chart relating to adaptive learning of anair-fuel sensor;

FIG. 9 is a high level flow chart for calling sensor diagnostics;

FIG. 10 is a high level flow chart for estimating catalyst temperaturedepending on engine operating mode;

FIG. 11 is a high level flow chart for performing default operation inresponse to sensor degradation;

FIG. 12 is a high level flow chart for disabling certain engineoperating modes;

FIGS. 13A-B are high level flow charts for controlling enginetransitions into catalyst heating modes;

FIG. 13C is a graphical representation of engine operating parametersduring transitions into and out of a catalyst heating mode;

FIG. 13D is a high level flow chart for controlling the engine out ofcatalyst heating mode;

FIGS. 13E-F are high level flow charts for controlling engine error-fuelratio during catalyst heating mode;

FIGS. 13G(1)-(3) illustrate engine operation during engine modetransitions;

FIG. 13H is a high level flow chart for controlling engine idle speedcontrol depending on whether catalyst heating is in progress;

FIG. 13I graphically represents operation according to an aspect of thepresent invention;

FIG. 13J graphically illustrates the effect of throttle position onengine air flow;

FIG. 13K is a high level flow chart for controlling engine idle speed;

FIG. 14 is a high level flow chart for adjusting ignition timing of theengine;

FIG. 15 is a high level flow chart for adjusting injected fuel based onoperating modes.

DETAILED DESCRIPTION

FIGS. 1A and 1B show one cylinder of a multi-cylinder engine, as well asthe intake and exhaust path connected to that cylinder. As describedlater herein with particular reference to FIG. 2, there are variousconfigurations of the cylinders and exhaust system.

Continuing with FIG. 1A, direct injection spark ignited internalcombustion engine 10, comprising a plurality of combustion chambers, iscontrolled by electronic engine controller 12. Combustion chamber 30 ofengine 10 is shown including combustion chamber walls 32 with piston 36positioned therein and connected to crankshaft 40. A starter motor (notshown) is coupled to crankshaft 40 via a flywheel (not shown). In thisparticular example, piston 36 includes a recess or bowl (not shown) tohelp in forming stratified charges of air and fuel. Combustion chamber,or cylinder, 30 is shown communicating with intake manifold 44 andexhaust manifold 48 via respective intake valves 52 a and 52 b (notshown), and exhaust valves 54 a and 54 b (not shown). Fuel injector 66Ais shown directly coupled to combustion chamber 30 for deliveringinjected fuel directly therein in proportion to the pulse width ofsignal fpw received from controller 12 via conventional electronicdriver 68. Fuel is delivered to fuel injector 66A by a conventional highpressure fuel system (not shown) including a fuel tank, fuel pumps, anda fuel rail.

Intake manifold 44 is shown communicating with throttle body 58 viathrottle plate 62. In this particular example, throttle plate 62 iscoupled to electric motor 94 so that the position of throttle plate 62is controlled by controller 12 via electric motor 94. This configurationis commonly referred to as electronic throttle control (ETC), which isalso utilized during idle speed control. In an alternative embodiment(not shown), which is well known to those skilled in the art, a bypassair passageway is arranged in parallel with throttle plate 62 to controlinducted airflow during idle speed control via a throttle control valvepositioned within the air passageway.

Exhaust gas sensor 76 is shown coupled to exhaust manifold 48 upstreamof catalytic converter 70 (note that sensor 76 corresponds to variousdifferent sensors, depending on the exhaust configuration. For example,it could correspond to sensor 230, or 234, or 230 b, or 230 c, or 234 c,or 230 d, or 234 d, as described in later herein with reference to FIG.2). Sensor 76 (or any of sensors 230, 234, 230 b, 230 c, 230 d, or 234d) may be any of many known sensors for providing an indication ofexhaust gas air/fuel ratio such as a linear oxygen sensor, a two-stateoxygen sensor, or an HC or CO sensor. In this particular example, sensor76 is a two-state oxygen sensor that provides signal EGO to controller12 which converts signal EGO into two-state signal EGOS. A high voltagestate of signal EGOS indicates exhaust gases are rich of stoichiometryand a low voltage state of signal EGOS indicates exhaust gases are leanof stoichiometry. Signal EGOS is used to advantage during feedbackair/fuel control in a conventional manner to maintain average air/fuelat stoichiometry during the stoichiometric homogeneous mode ofoperation.

Conventional distributorless ignition system 88 provides ignition sparkto combustion chamber 30 via spark plug 92 in response to spark advancesignal SA from controller 12.

Controller 12 causes combustion chamber 30 to operate in either ahomogeneous air/fuel mode or a stratified air/fuel mode by controllinginjection timing. In the stratified mode, controller 12 activates fuelinjector 66A during the engine compression stroke so that fuel issprayed directly into the bowl of piston 36. Stratified air/fuel layersare thereby formed. The strata closest to the spark plug contains astoichiometric mixture or a mixture slightly rich of stoichiometry, andsubsequent strata contain progressively leaner mixtures. During thehomogeneous mode, controller 12 activates fuel injector 66A during theintake stroke so that a substantially homogeneous air/fuel mixture isformed when ignition power is supplied to spark plug 92 by ignitionsystem 88. Controller 12 controls the amount of fuel delivered by fuelinjector 66A so that the homogeneous air/fuel mixture in chamber 30 canbe selected to be at stoichiometry, a value rich of stoichiometry, or avalue lean of stoichiometry. The stratified air/fuel mixture will alwaysbe at a value lean of stoichiometry, the exact air/fuel being a functionof the amount of fuel delivered to combustion chamber 30. An additionalsplit mode of operation wherein additional fuel is injected during theexhaust stroke while operating in the stratified mode is also possible.

Nitrogen oxide (NOx) adsorbent or trap 72 is shown positioned downstreamof catalytic converter 70. NOx trap 72 is a three-way catalyst thatabsorbs NOx when engine 10 is operating lean of stoichiometry. Theabsorbed NOx is subsequently reacted with HC and CO and catalyzed whencontroller 12 causes engine 10 to operate in either a rich homogeneousmode or a near stoichiometric homogeneous mode such operation occursduring a NOx purge cycle when it is desired to purge stored NOx from NOxtrap 72, or during a vapor purge cycle to recover fuel vapors from fueltank 160 and fuel vapor storage canister 164 via purge control valve168, or during operating modes requiring more engine power, or duringoperation modes regulating temperature of the omission control devicessuch as catalyst 70 or NOx trap 72. (Again, note that emission controldevices 70 and 72 can correspond to various devices described in FIG. 2.For example, they can correspond to devices 220 and 224, 220 b and 224b, etc.).

Controller 12 is shown in FIG. 1A as a conventional microcomputer,including microprocessor unit 102, input/output ports 104, an electronicstorage medium for executable programs and calibration values shown asread only memory chip 106 in this particular example, random accessmemory 108, keep alive memory 110, and a conventional data bus.Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, includingmeasurement of inducted mass air flow (MAF) from mass air flow sensor100 coupled to throttle body 58; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a profile ignitionpickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft40; and throttle position TP from throttle position sensor 120; andabsolute Manifold Pressure Signal MAP from sensor 122. Engine speedsignal RPM is generated by controller 12 from signal PIP in aconventional manner and manifold pressure signal MAP from a manifoldpressure sensor provides an indication of vacuum, or pressure, in theintake manifold. During stoichiometric operation, this sensor can giveand indication of engine load. Further, this sensor, along with enginespeed, can provide an estimate of charge (including air) inducted intothe cylinder. In a preferred aspect of the present invention, sensor118, which is also used as an engine speed sensor, produces apredetermined number of equally spaced pulses every revolution of thecrankshaft.

In this particular example, temperature Tcat of catalytic converter 70and temperature Ttrp of NOx trap 72 are inferred from engine operationas disclosed in U.S. Pat. No. 5,414,994, the specification of which isincorporated herein by reference. In an alternate embodiment,temperature Tcat is provided by temperature sensor 124 and temperatureTtrp is provided by temperature sensor 126.

Continuing with FIG. 1A, camshaft 130 of engine 10 is showncommunicating with rocker arms 132 and 134 for actuating intake valves52 a, 52 b and exhaust valve 54 a. 54 b. Camshaft 130 is directlycoupled to housing 136. Housing 136 forms a toothed wheel having aplurality of teeth 138. Housing 136 is hydraulically coupled to an innershaft (not shown), which is in turn directly linked to camshaft 130 viaa timing chain (not shown). Therefore, housing 136 and camshaft 130rotate at a speed substantially equivalent to the inner camshaft. Theinner camshaft rotates at a constant speed ratio to crankshaft 40.However, by manipulation of the hydraulic coupling as will be describedlater herein, the relative position of camshaft 130 to crankshaft 40 canbe varied by hydraulic pressures in advance chamber 142 and retardchamber 144. By allowing high pressure hydraulic fluid to enter advancechamber 142, the relative relationship between camshaft 130 andcrankshaft 40 is advanced. Thus, intake valves 52 a, 52 b and exhaustvalves 54 a, 54 b open and close at a time earlier than normal relativeto crankshaft 40. Similarly, by allowing high pressure hydraulic fluidto enter retard chamber 144, the relative relationship between camshaft130 and crankshaft 40 is retarded. Thus, intake valves 52 a, 52 b, andexhaust valves 54 a, 54 b open and close at a time later than normalrelative to crankshaft 40.

Teeth 138, being coupled to housing 136 and camshaft 130, allow formeasurement of relative cam position via cam timing sensor 150 providingsignal VCT to controller 12. Teeth 1, 2, 3, and 4 are preferably usedfor measurement of cam timing and are equally spaced (for example, in aV-8 dual bank engine, spaced 90 degrees apart from one another) whiletooth 5 is preferably used for cylinder identification, as describedlater herein. In addition, controller 12 sends control signals (LACT,RACT) to conventional solenoid valves (not shown) to control the flow ofhydraulic fluid either into advance chamber 142, retard chamber 144, orneither.

Relative cam timing is measured using the method described in U.S. Pat.No. 5,548,995, which is incorporated herein by reference. In generalterms, the time, or rotation angle between the rising edge of the PIPsignal and receiving a signal from one of the plurality of teeth 138 onhousing 136 gives a measure of the relative cam timing. For theparticular example of a V-8 engine, with two cylinder banks and afive-toothed wheel, a measure of cam timing for a particular bank isreceived four times per revolution, with the extra signal used forcylinder identification.

Sensor 160 provides an indication of both oxygen concentration in theexhaust gas as well as NOx concentration. Signal 162 provides controllera voltage indicative of the O2 concentration while signal 164 provides avoltage indicative of NOx concentration.

As described above, FIGS. 1A (and 1B) merely shows one cylinder of amulti-cylinder engine, and that each cylinder has its own set ofintake/exhaust valves, fuel injectors, spark plugs, etc.

Referring now to FIG. 1B, a port fuel injection configuration is shownwhere fuel injector 66B is coupled to intake manifold 44, rather thandirectly cylinder 30.

Also, in each embodiment of the present invention, the engine is coupledto a starter motor (not shown) for starting the engine. The startermotor is powered when the driver turns a key in the ignition switch onthe steering column, for example. The starter is disengaged after enginestart as evidence, for example, by engine 10 reaching a predeterminedspeed after a predetermined time. Further, in each embodiment, anexhaust gas recirculation (EGR) System routes a desired portion ofexhaust gas from exhaust manifold 48 to intake manifold 44 via an EGRvalve (not shown). Alternatively, a portion of combustion gases may beretained in the combustion chambers by controlling exhaust valve timing.

The engine 10 operates in various modes, including lean operation, richoperation, and “near stoichiometric” operation. “Near stoichiometric”operation refers to oscillatory operation around the stoichiometric airfuel ratio. Typically, this oscillatory operation is governed byfeedback from exhaust gas oxygen sensors. In this near stoichiometricoperating mode, the engine is operated within one air-fuel ratio of thestoichiometric air-fuel ratio.

As described below, feedback air-fuel ratio is used for providing thenear stoichiometric operation. Further, feedback from exhaust gas oxygensensors can be used for controlling air-fuel ratio during lean andduring rich operation. In particular, a switching type, heated exhaustgas oxygen sensor (HEGO) can be used for stoichiometric air-fuel ratiocontrol by controlling fuel injected (or additional air via throttle orVCT) based on feedback from the HEGO sensor and the desired air-fuelratio. Further, a UEGO sensor (which provides a substantially linearoutput versus exhaust air-fuel ratio) can be used for controllingair-fuel ratio during lean, rich, and stoichiometric operation. In thiscase, fuel injection (or additional air via throttle or VCT) is adjustedbased on a desired air-fuel ratio and the air-fuel ratio from thesensor. Further still, individual cylinder air-fuel ratio control couldbe used if desired.

Also note that various methods can be used according to the presentinvention to maintain the desired torque such as, for example, adjustingignition timing, throttle position, variable cam timing position, andexhaust gas recirculation amount. Further, these variables can beindividually adjusted for each cylinder to maintain cylinder balanceamong all the cylinder groups. Engine torque control is described morespecifically herein in FIGS. 3A-C, 4C, and others such as 13J, K.

Referring now to FIGS. 2A-2D, various configurations that can be usedaccording to the present invention are described. In particular, FIG. 2Adescribes an engine 10 having a first group of cylinders 210 and asecond group of cylinders 212. In this particular example, first andsecond groups 210 and 212 have four combustion chambers each. However,the groups can have different numbers of cylinders including just asingle cylinder. And engine 10 need not be a V-engine, but also may bean in-line engine where the cylinder grouping do not correspond toengine banks. Further, the cylinder groups need not include the samenumber of cylinders in each group.

First combustion chamber group 210 is coupled to the first catalyticconverter 220. Upstream of catalyst 220 and downstream of the firstcylinder group 210 is an exhaust gas oxygen sensor 230. Downstream ofcatalyst 220 is a second exhaust gas sensor 232.

Similarly, second combustion chamber group 212 is coupled to a secondcatalyst 222. Upstream and downstream are exhaust gas oxygen sensors 234and 236 respectively. Exhaust gas spilled from the first and secondcatalyst 220 and 222 merge in a Y-pipe configuration before enteringdownstream under body catalyst 224. Also, exhaust gas oxygen sensors 238and 240 are positioned upstream and downstream of catalyst 224,respectively.

In one example embodiment, catalysts 220 and 222 are platinum andrhodium catalysts that retain oxidants when operating lean and releaseand reduce the retained oxidants when operating rich. Similarly,downstream underbody catalyst 224 also operates to retain oxidants whenoperating lean and release and reduce retained oxidants when operatingrich. Downstream catalyst 224 is typically a catalyst including aprecious metal and alkaline earth and alkaline metal and base metaloxide. In this particular example, downstream catalyst 224 containsplatinum and barium. Also, various other emission control devices couldbe used in the present invention, such as catalysts containing palladiumor perovskites. Also, exhaust gas oxygen sensors 230 to 240 can besensors of various types. For example, they can be linear oxygen sensorsfor providing an indication of air-fuel ratio across a broad range.Also, they can be switching type exhaust gas oxygen sensors that providea switch in sensor output at the stoichiometric point. Further, thesystem can provide less than all of sensors 230 to 240, for example,only sensors 230, 234, and 240.

When the system of FIG. 2A is operated in the AIR/LEAN mode, firstcombustion group 210 is operated without fuel injection and secondcombustion group 212 is operated at a lean air-fuel ratio (typicallyleaner than about 18:1). Thus, in this case, and during this operation,sensors 230 and 232 see a substantially infinite air-fuel ratio.Alternatively, sensors 234 and 236 see essentially the air-fuel ratiocombusted in the cylinders of group 212 (other than for delays andfiltering provided by the storage reduction catalysts 222). Further,sensors 238 and 240 see a mixture of the substantially infinite air-fuelratio from the first combustion chamber 210 and the lean air-fuel ratiofrom the second combustion chamber group 212.

As described later herein, diagnosis of sensors 230 and 232 can beperformed when operating in the AIR/LEAN mode if the sensors indicate anair-fuel ratio other than lean. Also, diagnostics of catalysts 220 and222 are disabled when operating in the AIR/LEAN mode in the system ofFIG. 2A, since the catalysts do not see a varying air-fuel ratio.

Referring now to FIG. 2B, engine 10B is shown with first and secondcylinder groups 210 b and 212 b. In this example, an inline fourcylinder engine is shown where the combustion chamber groups are equallydistributed. However, as described above herein with particularreference to FIG. 2A, the combustion chamber groups do not need to haveequal number of cylinders. In this example, exhaust gases from bothcylinder groups 210 b and 212 b merge in the exhaust manifold. Engine10B is coupled to catalysts 220 b. Sensors 230 b and 232 b arepositioned upstream and downstream of the upstream catalyst 220 b.Downstream catalyst 224 b is coupled to catalyst 222 b. In addition, athird exhaust gas oxygen sensor 234 b is positioned downstream ofcatalyst 224 b.

With regard to FIG. 2B, when the engine is operating in the AIR/LEANmode, regardless of which cylinder group is operating lean and which isoperating without fuel injection, all of the exhaust gas oxygen sensorsand catalysts see a mixture of gases having a substantially infiniteair-fuel ratio from group 210B and gases having a lean air-fuel ratiofrom group 212 b.

Referring now to FIG. 2C, a system similar to FIG. 2A is shown. However,in FIG. 2C, the cylinder groups 210 c and 212 c are distributed acrossengine banks so that each bank has some cylinders in a first group andsome cylinders in a second group. Thus, in this example, two cylindersfrom group 210 c and two cylinders from group 212 c are coupled tocatalysts 220 c. Similarly, two cylinders from group 210 c and 212 c arecoupled to catalysts 222 c.

In the system of FIG. 2C, when the engine is operating in the AIR/LEANmode, all of the sensors (230 c to 240 c) and all of the catalysts (220c to 224 c) see a mixture of gases having a substantially infiniteair-fuel ratio and gases having a lean air-fuel ratio as previouslydescribed with particular reference to FIG. 2A.

Referring now to FIG. 2D, yet another configuration is described. Inthis example, the first and second cylinder groups 210 d and 212 d havecompletely independent exhaust gas paths. Thus, when the engine isoperating in the AIR/LEAN mode, the cylinder group 210 d withoutinjected fuel, sensors 230 d, 232 d, and 238 d all see a gas withsubstantially infinitely lean air-fuel ratio. Alternatively, sensors 234d, 236 d, and 240 d see a lean exhaust gas mixture (other than delay andfiltering effects of catalysts 222 d and 226 d).

In general, the system of FIG. 2C is selected for a V-8 engine, whereone bank of the V is coupled to catalyst 220 c and the other bank iscoupled to catalyst 222 c, with the first and second cylinder groupsbeing indicated by 210 c and 212 c. However, with a V-10 engine,typically the configuration of FIG. 2A or 2D is selected.

Referring now to FIGS. 2E-2H, various fuel delivery and air/fuel modesof operation are described. These modes of operation include feedbackcorrection to the fuel delivered in response to one or more exhaust gasoxygen sensors coupled to the exhaust of engine 10. These modes alsoinclude various adaptive learning modes including: adaptively learningerrors caused by either inducting air or delivering fuel into engine 10;adaptively learning fuel vapor concentration of fuel vapors inductedinto engine 10; and adaptively learning the fuel mixture of a multi-fuelengine such as an engine adapted to operate on a blend of fuel andalcohol.

Referring now to FIG. 2E, closed loop, or feedback, fuel control isenabled in block 1220 when certain engine operating conditions are metsuch as sufficient engine operating temperature. First, the operationdescribed in FIG. 2E proceeds, if not in the AIR/LEAN mode (block 1218).If in the AIR/LEAN mode, air/fuel control is provided in FIG. 5. Whennot in AIR/LEAN mode and when in closed loop fuel control, the desiredair-fuel ratio (A/Fd) is first determined in step 1222. Desired A/Fd maybe a stoichiometric air-fuel mixture to achieve low emissions byoperating essentially within the peak efficiency window of a three-waycatalyst. Desired A/Fd also may be an overall air-fuel mixture lean ofstoichiometry to achieve improved fuel economy, and desired A/Fd may berich of stoichiometry when either acceleration is required or fastercatalyst warm up is desired.

In block 1224, desired fuel Fd is generated from the following equation:$\frac{{Fd} = {{MAF} \cdot {Ka}}}{{A/{Fd}} \cdot {FV}} - {VPa}$

where:

MAF is an indication of the mass airflow inducted into engine 10 whichmay be derived from either a mass airflow meter, or from a commonlyknown speed density calculation responsive to an indication of intakemanifold pressure;

Ka is an adaptively learned term to correct for long term errors in theactual air-fuel ratio such as may be caused by a faulty mass airflowmeter, an inaccurate fuel injector, or any other cause for error ineither airflow inducted into engine 10 or fuel injected into engine 10.Regeneration of Ka is described in greater detail later herein withparticular reference to FIG. 2F;

FV is a feedback variable derived from one or more exhaust gas oxygensensors. Its generation is described in more detail later herein withparticular reference to FIG. 2E;

VPa is an adaptively learned correction to compensate for fuel vaporsinducted into engine 10, its generation is described in greater detaillater herein with particular reference to FIG. 2G.

Desired fuel quantity Fd is then converted to a desired fuel pulse widthin block 1226 for driving those fuel injectors enabled to deliver fuelto engine 10.

Steps 1228-1240 of FIG. 2E describe in general a proportional plusintegral feedback controller for generating feedback variable FV inresponse to one or more exhaust gas sensors. Integral term Δi andproportional term Pi are determined in step 1228. Although only oneintegral and one proportional term are shown herein, different terms maybe used when making corrections in the lean direction than those termsused when making corrections in the rich direction so as to provide anoverall air-fuel bias. In step 1230, an overall output of the exhaustgas oxygen sensor designated as EGO is read and compared with desiredA/Fd. Signal EGO may be a simple two state representation of either alean air-fuel mixture or a rich air-fuel mixture. Signal EGO may also bea representation of the actual air-fuel mixture in engine 10. Further,signal EGO may be responsive to only to one exhaust gas oxygen sensorpositioned upstream of the three-way catalytic converters. And, signalEGO may be responsive to both exhaust gas oxygen sensors positionedupstream and downstream of the three-way catalytic converter.

When signal EGO is greater than desired A/Fd (block 1230), and it wasalso greater than A/Fd during the previous sample, (Block 1232),feedback variable FV is decremented by integral value Δi (block 234).Stated another way, when the exhaust gases are indicated as being lean,and were also lean during the previous sample period, signal FV isdecremented to provide a rich correction to delivered fuel. Conversely,when signal EGO is greater than desired A/Fd (block 1230), but was notgreater than A/Fd (block 1232) during the previous sample, proportionalterm Pi is subtracted from feedback variable FV (block 1236). That is,when exhaust gases change from rich to lean, a rapid rich correction ismade by decrementing proportional value Pi from feedback variable FV.

On the other hand, when signal EGO is less than A/Fd (block 1230),indicating exhaust gases are rich, and the exhaust gases were richduring the previous sample period (block 1238), integral term Δi isadded to feedback variable FV (block 1242). However, when exhaust gasesare rich (block 1230), but were previously lean (block 1238),proportional term Pi is added to feedback variable FV (block 1240).

It is noted that in this particular example, feedback variable FV occursin the denominator of the fuel delivery equation (block 1224).Accordingly, a lean air-fuel correction is made when feedback variableFV is greater than unity, and a rich correction is made when signal FVis less than unity. In other examples, a feedback variable may occur inthe numerator, so that opposite corrections would be made.

Note that various other air-fuel feedback control methods can be used,such as state-space control, nonlinear control, or others.

Referring now to FIG. 2F, a routine for adaptively learning a correctionvalue for air-fuel ratio errors caused by degraded components, such asfaulty airflow meters or faulty fuel injectors, is now described. Afterit is determined that operation is not in the AIR/LEAN Mode (block 248),and adaptive learning of long-term air-fuel errors is desired (block1250), and closed loop fuel control is enabled (block 1252), adaptivelearning of fuel vapor concentration is disabled in block 1254. Thedesired air-fuel ratio A/Fd is then set to the stoichiometric value inblock 1258. When feedback value FV is greater than unity (block 1260),or other indications are given that a lean fuel correction is desiredbecause engine 10 is operating too rich, adaptive term Ka is decrementedin block 1264. That is, a lean correction to delivered fuel (see block1224 of FIG. 2E) is provided when it is apparent that engine 10 isoperating too rich and feedback air-fuel control FV is continuouslyproviding lean corrections. On the other hand, when feedback control isindicating that rich fuel corrections are being provided (block 1260),adaptive term Ka is incremented in block 266. That is, when feedbackcontrol is continuously providing rich corrections, adaptive term Ka isincremented to provide those rich corrections.

Referring now to FIG. 2G, adaptive learning of the concentration of fuelvapors inducted into engine 10 is now described. As discussed previouslyherein, fuel vapors are inducted from fuel tank 160 and fuel vaporstorage canister 164 into intake manifold 44 via vapor purge controlvalve 168. In this description, the generation of adaptive correctionvalue VPa is provided for correcting delivered fuel to compensate forfuel vapors being inducted into engine 10. fuel vapor purge is enabled,for example, when an indication of ambient temperature exceeds athreshold, or a period of engine operation has elapsed without purging,or engine temperature exceeds a threshold, or engine operation hasswitched to a stoichiometric, rich or homogenous air/fuel mode.

When not in the AIR/LEAN mode (block 1268), and when fuel vapor purge isenabled (block 1270), and adaptive learning of fuel vapor concentrationis also enabled (block 1274), and closed loop fuel control is enabled(block 1276), adaptive learning of air-fuel errors provided by adaptiveterm Ka is disabled (block 1280).

At block 1282, signal FV is compared to unity to determine whether leanor rich air-fuel rich corrections are being made. In this particularexample, closed loop fuel control about a stoichiometric air-fuel ratiois utilized to generate feedback variable FV. The inventor recognizes,however, that any feedback control system may be utilized at anyair-fuel ratio to determine whether lean or rich air-fuel correctionsare being made in response to the induction of fuel vapors into engine10. Continuing with this particular example, when feedback variable FVis greater than unity (block 1282), indicating that lean air-fuelcorrections are being made, vapor adaptive term VPa is incremented inblock 1286. On the other hand, when feedback variable FV is less thanunity, indicating that rich air-fuel corrections are being made,adaptively learned vapor concentration term VPa is decremented in block1290.

In accordance with the above described operation with reference to FIG.2G, adaptive term VPa adaptively learns the vapor concentration ofinducted fuel vapors and this adaptive term is used to correct deliveredfuel in, for example, block 1224 of FIG. 2E.

Referring now to FIG. 2H, a description of adaptively learning the fuelblend mixture is now provided. For example, engine 10 may operate on anunknown mixture of gasoline and an alcohol such as methanol. Theadaptive learning routine that will now be described provides anindication of the actual fuel blend being used. Again, this adaptivelearning is responsive to one or more exhaust gas oxygen sensors.

When not in the AIR/LEAN mode, and when the fuel level of fuel tank haschanged (block 1290), and engine 10 is operating in closed loop fuelcontrol mode (block 1292), adaptive learning of air-fuel error byadaptive term Ka, and adaptive learning of fuel vapor concentration byadaptive term VPa is disabled in block 1294. Feedback variable FV isdetermined in block 1296 as previously described with particularreference to FIG. 2E. In response to feedback variable FV, the overallengine air-fuel ratio is determined and, accordingly, the fuel blendmixture is inferred (block 1298). Stated another way, the stoichiometricair-fuel mixture of any fuel blend is known. And, it is also known thatfeedback variable FV provides an indication of engine air-fuel ratio.For example, feedback variable FV provides an indication of astoichiometric air-fuel ratio for pure gasoline when FV is equal tounity. When FV is equal to 1.1, for example, the overall engine air-fuelratio would be 10% leaner than the stoichiometric air-fuel ratio forgasoline. Accordingly, the fuel blend is easily inferred from feedbackvariable FV in block 298.

Referring now to FIG. 3A, a routine is described for controlling engineoutput and transitioning between engine operating modes. First, in step310, the routine determines a desired engine output. In this particularexample, the desired engine output is a desired engine brake torque.Note that there are various methods for determining the desired engineoutput torque, such as based on a desired wheel torque and gear ratio,based on a pedal position and engine speed, based on a pedal positionand vehicle speed and gear ratio, or various other methods. Also notethat various other desired engine output values could be used other thanengine torque, such as: engine power or engine acceleration.

Next, in step 312, the routine makes a determination as to whether atthe current conditions the desired engine output is within apredetermined range. In this particular example, the routine determineswhether the desired engine output is less than a predetermined engineoutput torque and whether current engine speed is within a predeterminedspeed range. Note that various other conditions can be used in thisdetermination, such as: engine temperature, catalyst temperature,transition mode, transition gear ratio, and others. In other words, theroutine determines in step 312 which engine operating mode is desiredbased on the desired engine output and current operating conditions. Forexample, there may be conditions where based on a desired engine outputtorque and engine speed, it is possible to operate with less than allthe cylinders firing, however, due to other needs such as purging fuelvapors or providing manifold vacuum, it is desired to operate with allcylinders firing. In other words, if manifold vacuum falls below apredetermined value, the engine is transitioned to operating with allcylinders combusting injected fuel. Alternatively, the transition can becalled if pressure in the brake booster is below a predetermined value.

On the other hand, operation in the AIR/LEAN mode is permitted duringfuel vapor purge if temperature of the catalyst is sufficient to oxidizethe purged vapors which will pass through the non-combusting cylinders.

Continuing with FIG. 3A, when the answer to step 312 is yes, the routinedetermines in step 314 as to whether all cylinders are currentlyoperating. When answer to step 314 is yes, a transition is scheduled totransition from firing all cylinders to disabling some cylinders andoperating the remaining cylinders at a leaner air-fuel ratio than whenall the cylinders were firing. The number of cylinders disabled is basedon the desired engine output. The transition of step 316, in oneexample, opens the throttle valve and increases fuel to the firingcylinders while disabling fuel to some of the cylinders. Thus, theengine transitions from performing combustion in all of the cylinders tooperating in the hereinafter referred to AIR/LEAN MODE. In other words,to provide a smooth transition in engine torque, the fuel to theremaining cylinders is rapidly increased while at the same time thethrottle valve is opened. In this way, it is possible to operate withsome cylinders performing combustion at an air/fuel ratio leaner than ifall of the cylinders were firing. Further, those remaining cylindersperforming combustion operate at a higher engine load per cylinder thanif all the cylinders were firing. In this way, a greater air-fuel leanlimit is provided thus allowing the engine to operate leaner and obtainadditional fuel economy.

Next, in step 318, the routine determines an estimate of actual engineoutput based on the number of cylinders combusting air and fuel. In thisparticular example, the routine determines an estimate of engine outputtorque. This estimate is based on various parameters, such as: enginespeed, engine airflow, engine fuel injection amount, ignition timing andengine temperature.

Next, in step 320, the routine adjusts the fuel injection amount to theoperating cylinders so that the determined engine output approaches thedesired engine output. In other words, feedback control of engine outputtorque is provided by adjusting fuel injection amount to the subset ofcylinders that are carrying out combustion.

In this way, according to the present invention, it is possible toprovide rapid torque control by changing fuel injection amount duringlean combustion of less than all of the engine cylinders. The firingcylinders thereby operate at a higher load per cylinder resulting in anincreased air-fuel operating range. Additional air is added to thecylinders so that the engine can operate at this higher air-fuel ratiothereby providing improved thermal efficiency. As an added effect, theopening of the throttle to provide the additional air reduces enginepumping work, further providing an increase in fuel economy. As such,engine efficiency and fuel economy can be significantly improvedaccording to the present invention.

Returning to step 312 when the answer is no, the routine continues tostep 322 where a determination is made as to whether all cylinders arecurrently firing. When the answer to step 322 is no, the routinecontinues to step 324 where a transition is made from operating some ofthe cylinders to operating all of the cylinders. In particular, thethrottle valve is closed and fuel injection to the already firingcylinders is decreased at the same time as fuel is added to thecylinders that were previously not combusting in air-fuel mixture. Then,in step 326, the routine determines an estimate of engine output in afashion similar to step 318. However, in step 326, the routine presumesthat all cylinders are producing engine torque rather than in step 318where the routine discounted the engine output based on the number ofcylinders not producing engine output.

Finally, in step 328, the routine adjusts at least one of the fuelinjection amount or the air to all the cylinders so that the determinedengine output approaches a desired engine output. For example, whenoperating at stoichiometry, the routine can adjust the electronicthrottle to control engine torque, and the fuel injection amount isadjusted to maintain the average air-fuel ratio at the desiredstoichiometric value. Alternatively, if all the cylinders are operatinglean of stoichiometry, the fuel injection amount to the cylinders can beadjusted to control engine torque while the throttle can be adjusted tocontrol engine airflow and thus the air-fuel ratio to a desired leanair-fuel ratio. During rich operation of all the cylinders, the throttleis adjusted to control engine output torque and the fuel injectionamount can be adjusted to control the rich air-fuel ratio to the desiredair-fuel ratio.

FIG. 3A shows one example of engine mode scheduling and control. Variousothers can be used as is now described.

In particular, referring now to FIG. 3B, a graph is shown illustratingengine output versus engine speed. In this particular description,engine output is indicated by engine torque, but various otherparameters could be used, such as, for example: wheel torque, enginepower, engine load, or others. The graph shows the maximum availabletorque that can be produced in each of four operating modes. Note that apercentage of available torque, or other suitable parameters, could beused in place of maximum available torque. The four operating modes inthis embodiment include:

Operating some cylinders lean of stoichiometry and remaining cylinderswith air pumping through and substantially no injected fuel (note: thethrottle can be substantially open during this mode), illustrated asline 33 ba in the example presented in FIG. 3B;

Operating some cylinders at stoichiometry, and the remaining cylinderspumping air with substantially no injected fuel (note: the throttle canbe substantially open during this mode), shown as line 334 a in theexample presented in FIG. 3B;

Operating all cylinders lean of stoichiometry (note: the throttle can besubstantially open during this mode, shown as line 332 a in the examplepresented in FIG. 3B;

Operating all cylinders substantially at stoichiometry for maximumavailable engine torque, shown as line 330 a in the example presented inFIG. 3B.

Described above is one exemplary embodiment according to the presentinvention where an 8-cylinder engine is used and the cylinder groups arebroken into two equal groups. However, various other configurations canbe used according to the present invention. In particular, engines ofvarious cylinder numbers can be used, and the cylinder groups can bebroken down into unequal groups as well as further broken down to allowfor additional operating modes. For the example presented in FIG. 3B inwhich a V-8 engine is used, lines 336 a shows operation with 4 cylindersoperating with air and substantially no fuel, lines 334 a showsoperation with four cylinders operating at stoichiometry and fourcylinders operating with air, line 332 a shows 8 cylinders operatinglean, and line 33 a shows 8 cylinders operating at stoichiometry.

The above described graph illustrates the range of available torques ineach of the described modes. In particular, for any of the describedmodes, the available engine output torque is any torque less than themaximum amount illustrated by the graph. Also note that in any modewhere the overall mixture air-fuel ratio is lean of stoichiometry, theengine can periodically switch to operating all of the cylindersstoichiometric or rich. This is done to reduce the stored oxidants(e.g., NOx) in the emission control device(s). For example, thistransition can be triggered based on the amount of stored NOx in theemission control device(s), or the amount of NOx exiting the emissioncontrol device(s), or the amount of NOx in the tailpipe per distancetraveled (mile) of the vehicle.

To illustrate operation among these various modes, several examples ofoperation are described. The following are simply exemplary descriptionsof many that can be made, and are not the only modes of operationaccording to the present invention. As a first example, consideroperation of the engine along trajectory A. In this case, the engineinitially is operating with four cylinders lean of stoichiometry, andfour cylinders pumping air with substantially no injected fuel. Then, inresponse to operating conditions, it is desired to change engineoperation along trajectory A. In this case, it is desired to changeengine operation to operating with four cylinders operating atsubstantially stoichiometric combustion, and four cylinders pumping airwith substantially no injected fuel. In this case, additional fuel isadded to the combusting cylinders to decrease air-fuel ratio towardstoichiometry, and correspondingly increase engine torque.

As a second example, consider trajectory labeled B. In this case, theengine begins by operating with four cylinders combusting atsubstantially stoichiometry, and the remaining four cylinders pumpingair with substantially no injected fuel. Then, in response to operatingconditions, engine speed changes and is desired to increase enginetorque. In response to this, all cylinders are enabled to combust airand fuel at a lean air-fuel ratio. In this way, it is possible toincrease engine output, while providing lean operation.

As a third example, consider the trajectory labeled C. In this example,the engine is operating with all cylinders combusting at substantiallystoichiometry. In response to a decrease in desired engine torque, fourcylinders are disabled to provide the engine output.

Continuing with FIG. 3B, and lines 330-336 in particular, anillustration of the engine output, or torque, operation for each of thefour exemplary modes is now described. For example, at engine speed N1,line 330 shows the available engine output or torque output that isavailable when operating in the 8-cylinder stoichiometric mode. Asanother example, line 332 indicates the available engine output ortorque output available when operating in the 8-cylinder lean mode atengine speed N2. When operating in the 4-cylinder stoichiometric and4-cylinder air mode, line 334 shows the available engine output ortorque output available when operating at engine speed N3. And, finally,when operating in the 4-cylinder lean, 4-cylinder air mode, line 336indicates the available engine or torque output when operating at enginespeed N4.

Referring now to FIG. 3C, an alternative routine to FIG. 3A is describedfor selecting the engine mode. In this particular example, the routinerefers to selecting between 4-cylinder and 8-cylinder combustion, andbetween lean and stoichiometric combustion. However, the routine can beeasily adjusted for various other combinations and numbers of cylinders.Continuing with FIG. 3C, in step 340, the routine determines whether thescheduled/requested torque (TQ_SCHED) is less than the available torquein the 4-cylinder stoichiometric mode where four cylinders arecombusting at substantially stoichiometry, and the remaining fourcylinders are pumping air with substantially no injected fuel. Note thatengine torque is utilized as just one example according to the presentinvention. Various other methods could be used such as comparing wheeltorque, engine power, wheel power, load, or various others. Further, anadjustment factor (TQ_LO_FR) is used to adjust the maximum availabletorque in the 4-cylinder stoichiometric mode to leave extra controlauthority.

When the answer to step 340 is yes, the routine continues to step 342where torque modulation is requested by setting the flag(INJ_CUTOUT_FLG) is set to 1. In other words, when the answer to step340 is yes, the routine determines that the desired mode is to have fourcylinders combusting and four cylinders flowing air with substantiallyno injected fuel. Further, in step 342, the routine calls for thetransition routine (see FIG. 3D). Next in step 343, the injectors arecut out in four of the cylinders. Then, in step 344, the routinedetermines whether the requested torque is less than the maximumavailable torque that can be provided in the mode where four cylindersare operated lean of stoichiometry, and four cylinders flow air withsubstantially no injected fuel. In other words, the parameter TQ_SCHEDis compared to parameter (TQ_MAX_(—)4L×TQ_LO_FR). When the answer tostep 344 is yes, this indicates that lean operation is available and theroutine continues to step 346. In step 346, the desired air-fuel ratio(LAMBSE, which also corresponds to A/Fd) is set to a lean air-fuel ratiodetermined based on engine speed and engine load (LEAN_LAMBSE).

When the answer to step 344 is no, the routine continues to step 348where the desired air-fuel ratio is set to a stoichiometric value. Thus,according to this example embodiment of the present invention, it ispossible to select between the four cylinder lean and the four cylinderstoichiometric mode when it is possible to operate in a four cylindermode.

When the answer to step 340 is no, the routine continues to step 350. Instep 350, the routine determines whether the flag (INJ_CUTOUT_FLG) isequal to 1. In other words, when the current conditions indicate thatthe engine is operating in the four cylinder mode, the answer to step350 is yes. When the answer to step 350 is yes, the routine calls atransition routine described later in FIG. 3E and sets the flag to 0.Then, the routine continues to step 354 where the routine determineswhether the requested torque is less than the maximum available torquein the 8-cylinder lean mode (TQ_MAX_(—)8L). When the answer to step 354is yes, the routine continues to step 356. In other words, when it ispossible to meet the current engine torque request in 8-cylinder leanmode, then the desired air-fuel ratio (LAMBSE) is set to a desired leanair-fuel ratio based on engine speed and load in step 356.

Continuing with FIG. 3C, when the answer to step 354 is no, then theengine is operated in the 8-cylinder stoichiometric mode, and thedesired engine air-fuel ratio (LAMBSE) is set to a stoichiometric valueis step 358.

Referring now to FIG. 3D(1), an example of engine operation intransitioning from an 8-cylinder mode to a 4-cylinder mode is described.The graph 3D(1)a illustrates the timing of the change in the cylindermode from eight cylinders to four cylinders. Graph 3D(1)b illustratesthe change in throttle position. Graph 3D(1)e illustrates the change inignition timing (spark retard). Graph 3D(1)2 illustrates engine torque.In this example, the graphs show how, as throttle position is graduallyincreased, ignition timing is retarded in an amount so that enginetorque stays substantially constant. While the graph illustratesstraight lines, this is an idealized version of actual engine operation,which of course, will show some variation. Also note that the throttleposition and ignition timing movements described previously occur beforethe transition. Once the throttle position and ignition timing reachpredetermined values, the cylinder mode is changed and at this point,ignition timing is returned to optimal (MBT) timing. In this way, theengine cylinder mode transition is achieved with substantially no effecton engine torque variation.

Referring now to FIG. 3D(2), a routine is described for transitioningfrom 8-cylinder mode to 4-cylinder mode. In step 360, the routinedetermines whether the engine is currently operating in the 8-cylindermode. When the answer to step 360 is yes, the routine continues to step362. In step 362, the routine determines whether conditions indicate theavailability of four cylinder operation as described previously hereinin particular reference to FIG. 3C. While the answer to step 362 is yes,the routine increments a timer (IC_ENA_TMR). Then, in step 366, theroutine determines whether the timer is less than a preselected time(IC_ENA_TIM). This time can be adjusted to different predetermined timesbased on engine operating conditions. In one particular example, thetime can be set to a constant value of one second. Alternatively, thetime can be adjusted depending on whether the driver is tipping in ortipping out.

Continuing with FIG. 3D(2), when the answer to step 366 is yes, theroutine continues to step 368. In step 368, the routine calculates atorque ratio (TQ_ratio), spark_retard, and relative throttle position(TP_REL). In particular, a torque ratio is calculated based on thenumber of cylinders being disabled (in this case four) to the totalnumber of cylinders (in this case eight), and the current timer valueand timer limit value (IC_ENA_TIM). Further, the spark retard iscalculated as a function of the torque ratio. Finally, the relativethrottle position is calculated as a function of the torque ratio.Alternatively, when the answer to step 366 is no, the routine continuesto step 370. In step 370, the routine operates in the four cylinder modeand sets the spark retard to zero.

Note that the difference in the times t1 and t2 in FIG. 3D(1) correspondto the timer value limit (IC_ENA_TIM).

Referring now to FIG. 3D(3), graphs 3D(3)a 3D(3)d illustrate transitionsfrom the 4-cylinder mode to the 8-cylinder mode. In this case, at timet, the ignition timing and the number of cylinders is changed. Then,from time t1 to time t2 (which corresponds to the timer limit value) thethrottle position and the ignition timing are ramped, or graduallyadjusted, to approach optimal ignition timing while maintaining enginetorque substantially constant. Also note that three different responsesare provided at three different transition times as set by parameter(IC_ENA_TIM). Further, in the first two responses as labeled by a and bthe driver is, for example, only requesting a slight gradual increase inengine torque. However, in situation c, the driver is requesting a rapidincrease in engine torque. In these cases, the graphs illustrate theadjustment in throttle position and ignition timing and the change inthe number of cylinders, as well as the corresponding engine output.

Referring now to FIG. 3E, the routine describes the transition from fourcylinders to the 8-cylinder mode. First, in step 372, the routinedetermines whether the engine is currently operating in the fourcylinder mode. When the answer to step 372 is yes, the routine continuesto step 374, where it is determined whether it is required to operate inthe 8-cylinder mode as described above herein with particular referenceto FIG. 3C. When the answer to step 374 is yes, the routine continues tostep 376. In step 376, the routine increments the timer (IC_DIS_TMR) andenables all cylinders. Then, in step 378, the routine determines whetherthe timer value is less than or equal to the limit time (IC_DIS_TIM). Asdescribed above herein, this timer limit is adjusted to achievedifferent engine responses. When the answer to step 378 is yes, theroutine continues to step 380 where the torque ratio, spark retard andrelative throttle position are calculated as shown.

Referring now to FIG. 4A, a routine for controlling engine idle speed isdescribed. First, in step 410 a, a determination is made as to whetheridle speed control is required. In particular, the routine determineswhether engine speed is within a predetermined idle speed control range,whether the pedal position is depressed less than a predeterminedamount, whether vehicle speed is less than a predetermined value, andother indications that idle speed control is required. When the answerto step 410 a is yes, the routine determines a desired engine speed instep 412 a. This desired engine speed is based on various factors, suchas: engine coolant temperature, time since engine start, position of thegear selector (for example, a higher engine speed is usually set whenthe transmission is in neutral compared with in drive), and accessorystatus such as air-conditioning, and catalyst temperature. Inparticular, desired engine speed may be increased to provide additionalheat to increase temperature of the catalyst during engine warm upconditions.

Then, in step 414 a, the routine determines actual engine speed. Thereare various methods for determining actual engine speed. For example,engine speed can be measured from an engine speed sensor coupled to theengine crankshaft. Alternatively, engine speed can be estimated based onother sensors, such as a camshaft position sensor and time. Then, instep 416 a, the routine calculates a control action based on thedetermined desired speed and measured engine speed. For example, a feedforward plus feed back proportional/integral controller can be used.Alternatively, various other control algorithms can be used so that theactual engine speed approaches the desired speed.

Next, in step 418 a, the routine determines whether the engine iscurrently operating in the AIR/LEAN mode. When the answer to step 418 ais no, the routine continues to step 420 a.

Referring now to step 420 a, a determination is made as to whether theengine should transition to a mode with some cylinders operating leanand other cylinders operating without injected fuel, referred to asAIR/LEAN mode. This determination can be made based on various factors.For example, various conditions may be occurring where it is desired toremain with all cylinders operating such as, for example: fuel vaporpurging, adaptive air/fuel ratio learning, a request for higher engineoutput by the driver, operating all cylinders rich to release and reduceoxidants stored in the emission control device, to increase exhaust andcatalyst temperature to remove contaminants such as sulfur, operating toincrease or maintain exhaust gas temperature to control any emissioncontrol device to a desired temperature or to lower emission controldevice temperature due to over-temperature condition. In addition, theabove-described conditions may occur not only when all the cylinders areoperating or all the cylinders are operating at the same air/fuel ratio,but also under other operating conditions such as: some cylindersoperating at stoichiometry and others operating rich, some cylindersoperating without fuel and just air, and other cylinders operating rich,or conditions where some cylinders are operating at a first air/fuelratio and other cylinders are operating at a second different air/fuelratio. In any event, these conditions may require transitions out of, orprevent operation in, the AIR/LEAN operating mode.

Referring now to step 422 a of FIG. 4A, a parameter other than fuel tothe second cylinder group is adjusted to control engine output andthereby control engine speed. For example, if the engine is operatingwith all of the cylinder groups lean, then the fuel injected to all ofthe cylinder groups is adjusted based on the determined control action.Alternatively, if the engine is operating in a stoichiometric mode withall of the cylinders operating at stoichiometry, then engine output andthereby engine speed is adjusted by adjusting the throttle or an airbypass valve. Further, in the stoichiometric mode, the stoichiometricair/fuel ratio of all the cylinders is adjusted by individuallyadjusting the fuel injected to the cylinders based on the desiredair/fuel ratio and the measured air/fuel ratio from the exhaust gasoxygen sensor in the exhaust path.

Thus, according to the present invention, when operating in the AIR/LEANmode, idle speed control is accomplished by adjusting fuel to thecylinders that are combusting air and fuel and the remaining cylindersare operated without fuel and only air. Note, that the fuel adjustmentcan be achieved by changing the engine air-fuel ratio via a change incombusted fuel-either injected or inducted in vapor form. However, whenthis AIR/LEAN mode is not employed, idle speed control is accomplishedin one of the following or other various manners: adjusting airflow andoperating at stoichiometry with retarded ignition timing, operating somecylinders at a first air/fuel ratio and other cylinders at a secondair/fuel ratio and adjusting at least one of air or fuel to thecylinders, adjusting an idle bypass valve based on speed error, orvarious others.

When the answer to step 420 a is yes, the routine continues to step 424a and the engine is transitioned from operating all the cylinders tooperating in the AIR/LEAN mode with some of the cylinders operating leanand other cylinders operating without injected fuel. (see transitioningroutines below).

From step 424 a or when the answer to step 418 a is yes, the routinecontinues to step 426 a and idle speed is controlled while operating inthe AIR/LEAN mode. Referring now to step 426 a of FIG. 4a, the fuelprovided to the cylinder group combusting an air/fuel mixture isadjusted based on the determined control action. Thus, the engine idlespeed is controlled by adjusting fuel to less than all of the cylindergroups and operating with some cylinders having no injected fuel.Further, if it is desired to control the air/fuel ratio of thecombusting cylinders, or the overall air/fuel ratio of the mixture ofpure air and combusted air and fuel, based on, for example, an exhaustgas oxygen sensor, then the throttle is adjusted based on the desiredair/fuel ratio and the measured air/fuel ratio. In this way, fuel to thecombusting cylinders is adjusted to adjust engine output, while air/fuelratio is controlled by adjusting airflow. Note, in this way, thethrottle can be used to keep the air-fuel ratio of the combustingcylinders within a preselected range to provide good combustibility andreduced pumping work.

Thus, according to the present invention, when operating in the AIR/LEANmode, fuel injected to the cylinders combusting a lean air-fuel mixtureis adjusted so that actual engine speed approaches a desired enginespeed, while some of the cylinders operate without injected fuel.Alternatively, when the engine is not operating in the AIR/LEAN mode, atleast one of the air and fuel provided all the cylinders is adjusted tocontrol engine speed to approach the desired engine speed.

The above description of FIG. 4a referred to the embodiment for idlespeed control. However, this is simply one embodiment according to thepresent invention. FIGS. 4b through 4 d refer to additional alternateembodiments.

Referring now to FIG. 4B, an embodiment directed to cruise control(vehicle speed control) is described. In particular, the routine of FIG.4B is similar to that of FIG. 4A except for blocks 410 b through 416 b.In particular, in step 410 b, a determination is made as to whethercruise control mode is selected. When the answer to step 410 b is yes,the routine continues to step 412 b, where a desired vehicle speed isdetermined. In step 412 b, various methods are available for selectingthe desired vehicle speed. For example, this may be a vehicle speed setdirectly by the vehicle driver. Alternatively, it could be a desiredvehicle speed to give a desired vehicle acceleration or decelerationrequested by the vehicle driver via steering wheel controls. Next, instep 414 b, the routine calculates/estimates actual vehicle speed. Thisactual vehicle speed can be calculated/estimated in various methods,such as, for example: based on vehicle speed sensors, based on enginespeed and a gear ratio, based on a global positioning system, or variousother methods. Next, in step 416 b, the routine calculates a controlaction based on the desired and actual vehicle speed. As describedabove, various control methods can be used, such as, for example: a PIDcontroller, a feed-forward controller, or various others.

Referring now to FIG. 4C, another alternate embodiment is described forcontrolling engine, or wheel, torque during the AIR/LEAN mode. Again,FIG. 4c is similar to FIGS. 4A and B, except for steps 410 c through 416c. First, in step 410 c, the routine determines whether torque controlis selected. When the answer to step 410 c is yes, the routine continuesto step 412 c. In step 412 c, the routine determines a desired torque(either an engine torque, wheel torque, or another torque value). Inparticular, this desired torque value can be based on variousparameters, such as, for example: a driver request (pedal position), adesired engine speed, a desired vehicle speed, a desired wheel slip, orvarious other parameters. As such, this torque control routine can beused to accomplish idle speed control, cruise control, driver control,as well as traction control.

Next, in step 414 c, the routine calculates/estimates actual torque.This can be accomplished via a torque sensor, or based on other engineoperating parameters, such as engine speed, engine airflow, and fuelinjection, and others. Then, in step 416 c, the routine calculatescontrol action based on the desired and actual torque. As above, variouscontrol methodologies can be used, such as a PID controller.

Finally, in FIG. 4D, another embodiment is described which is directedto traction control. In step 410 d, the routine determines whethertraction control is activated. When the answer to step 410 d is yes, theroutine continues to step 412 d, where the routine determines a wheelslip limit. This limit represents the maximum allowed wheel slip betweendriving and driven wheels that is tolerated. Then, in step 414 d, theroutine calculates/estimates actual wheel slip based on, for example,wheel speed sensors on the driving and driven wheels. Then, in step 416d, the routine continues to calculate a control action based on thelimit wheel slip and the calculated/estimated wheel slip. As above, withregard to FIGS. 4A through 4C, steps 418 d through 426 d are similar tosteps 418 a through 426 a.

Referring now to FIG. 5, a routine is described for controlling engineair-fuel ratio according to the present invention. First, in step 510, adetermination is made as to whether the engine is operating in open loopor closed loop air-fuel ratio control. In particular, in one example,open loop air-fuel ratio operation is performed during engine start upuntil the exhaust gas oxygen sensors have reached their operatingtemperature. Also, open loop air-fuel ratio control may be required whenoperating away from stoichiometry in the case where the exhaust gasoxygen sensors are switching type exhaust gas oxygen sensors thatprovide a switch and sensor output at the stoichiometric point. When theengine is operating in the open loop air-fuel ratio control mode, theroutine simply ends. Otherwise, when operating in the closed loop mode,the routine continues to step 512, where all of the exhaust gas oxygensensors coupled to the engine exhaust are read. Also note that operationin the AIR/LEAN operating mode may be prohibited when conditions aresuch that open loop control is required. However, it is also possible toprovide the AIR/LEAN mode in the open loop mode.

Next, in step 514, a determination is made as to whether the engine isoperating in the AIR/LEAN mode. When the answer to step 514 is yes, theroutine continues to step 516. In step 516, a determination is made, foreach sensor, as to whether the sensor is exposed to a mixture of air andcombusted fuel (i.e., whether the sensor sees a mixture of gases from afirst cylinder group with substantially no fuel injection and gases froma second combustion chamber group performing combustion of an air andfuel mixture). When the answer to step 516 is no, then it is notnecessary to take into account the mixture of pure air and combustedgases in utilizing information from the sensor. As such, the routine cancontinue to step 522 where air/fuel control is provided as shown in FIG.2E and the corresponding written description. Alternatively, if theanswer to step 516 is yes, the routine continues to step 518. As such,if the sensor is exposed to a mixture of air and combusted air and fuel,the routine continues to step 518.

In step 518, a determination is made as to whether the sensor is used tocontrol air-fuel ratio of cylinders combusting an air and fuel mixture.In other words, a sensor such as 230B for example, can be exposedthrough a mixture of air and combusted air and fuel and still be used tocontrol air-fuel ratio of the combusting cylinder group, in this example212B. When the answer to step 518 is no, the routine continues to step522 as described below herein. Alternatively, when the answer to step518 is yes, the routine continues to step 520. In step 520, the routinecorrects the combustion air fuel mixture for the sensor reading byadjusting one of air or fuel or both provided to the combustingcylinders based on the number of cylinders combusting the mixture andthe number of cylinders operating without substantial fuel injection,thereby taking into account the mixture of pure air and combusted gases.Stated another way, the routine corrects for the sensor offset caused bypure air from the combustion group (for example 210B) that has inductedair, but no injected fuel. In addition, the routine can take intoaccount recycled exhaust gas in the exhaust passage and intake passageif present. For example, if operating with the configuration of FIG.2(C), the upstream sensors see a mixture of air and combusted gasses. Assuch, the raw sensor reading does not correspond to the air-fuel ratioof the combusted gasses. According to the present invention, this erroris compensated in various ways.

In one particular example, the air-fuel ratio of the combustingcylinders can be determined from the sensor reading as shown below. Inthis example, an assumption of perfect mixing in the exhaust gas ismade. Further, it is assumed that the cylinders combusting air-fuelmixture all are combusting substantially the same air-fuel ratio. Inthis example, the sensor reading(s) is provided in terms of a relativeair-fuel ratio through stoichiometry. For gasoline, this ratio isapproximately 14.6. The air per cylinder for cylinders without injectedfuel is denoted as a_(A). Similarly, the air per cylinder for combustingcylinders is denoted as a_(C), while the fuel injected per cylinder forthe combustion cylinders is denoted as s_(C). The number of cylinderswithout fuel injected is denoted as Na, while the number of cylinderscombusting an air fuel mixture is denoted as N_(C). The general equationto relate these parameters is: $\begin{matrix}{{S \cdot 14.6} = \frac{{N_{A} \cdot a_{A}} + {N_{C} \cdot a_{C}}}{N_{C} \cdot f_{C}}} & {{Equation}\quad 1}\end{matrix}$

Assuming that the air provided to each combustion chamber group issubstantially the same, then the air to fuel ratio of the combustioncylinders can be found from multiplying the sensor reading by 14.6 andthe number of cylinders combusting an air-fuel mixture divided by thetotal number of cylinders. In the simple case where equal number ofcylinders operate with and without fuel, the sensor simply indicatestwice the combustion air-fuel ratio.

In this way, it is possible to utilize the sensor reading that iscorrupted by air from the cylinders without fuel injection. In thisexample, the sensor reading was modified to obtain an estimate of theair fuel ratio combusted in the combustion cylinders. Then this adjustedsensor reading can be used with a feedback control to control thecylinder air-fuel ratio of the combustion cylinders to a desiredair-fuel ratio taking into account the air affecting the sensor outputfrom the cylinders without fuel injection.

In an alternative embodiment to the present invention, the desiredair-fuel ratio can be adjusted to account for the air affecting thesensor output from the cylinders without fuel injection. In thisalternative embodiment, the sensor reading is not adjusted directly,rather the desired air-fuel ratio is adjusted accordingly. In this wayit is possible to control the actual air-fuel ratio in the cylinderscombusting an air-fuel mixture to a desired air-fuel ratio despite theeffect of the air from the cylinders without fuel injection on thesensor output.

In a similar manner, it is possible to account for the recycled exhaustgas. In other words, when operating lean, there is excess air in therecycled exhaust gas that enters the engine unmeasured by the air meter(air flow sensor 100). The amount of excess air in the EGR gasses(Am_egr) can be calculated from the equation below, using the measuredair mass from sensor 100 (am, in lbs/min), the EGR rate, or percent,(egrate), and the desired relative air-fuel ratio to stoichiometry(lambse):

am _(—) egr=am*(egrate/(1−egrate))*(lambse−1)

Where egrate=100*desem/(am+desem), where desem is the mass of EGR inlbs/min.

Thus, the corrected air mass would be am+am_egr.

In this way, it is possible to determine the actual air entering theengine cylinder so that air-fuel ratio can be controlled moreaccurately.

In other words, if operating in open loop fuel control, the excess airadded through the EGR will operate the cylinder leaner than requestedand could cause lean engine mis-fires if not accounted for. Similarly,if operating in closed loop fuel control, the controller may adjust thedesired air-fuel ratio such that more fuel is added to make the overallair-fuel ratio match the requested value. This may cause engine outputto be off proportional to the value am_egr. The solution to these is to,for example, adjust the requested air mass by reducing the requestedairflow from the electronically controlled throttle by an amount ofam_egr so as to maintain the engine output and air-fuel ratio.

Note that in some of the above corrections, the adjustments made tocompensate for the uncombusted air in some cylinders requires anestimate of airflow in the cylinders. However, this estimate may havesome error (for example, if based on an airflow sensor, there may be asmuch as 5% error, or more). Thus, the inventors have developed anothermethod for determining air-fuel ratio of the combusted mixture. Inparticular, using temperature sensor coupled to an emission controldevice (e.g., 220 c), it is possible to detect when the operatingcylinders have transitioned through the stoichiometric point. In otherwords, when operating the combusting cylinders lean, and other cylinderswith substantially no injected fuel, there will be almost no exothermicreaction across the catalyst since only excess oxygen is present (andalmost no reductants are present since no cylinders are operating rich).As such, catalyst temperature will be at an expected value for currentoperating conditions. However, if the operating cylinders transition toslightly rich of stoichiometry, the rich gasses can react with theexcess oxygen across the catalyst, thereby generating heat. This heatcan raise catalyst temperature beyond that expected and thus it ispossible to detect the combustion air-fuel ratio from the temperaturesensor. This correction can be used with the above described methods forcorrecting the air-fuel ratio reading so that accurate air-fuel ratiofeedback control can be accomplished when operating some cylinders withsubstantially no injected fuel.

Continuing with FIG. 5, in step 522, the air-fuel ratio of the cylinderscarrying out a combustion is corrected based on the output of thesensors read in step 512. In this case, since the engine is notoperating in the AIR/LEAN mode, it is generally unnecessary to correctthe sensor outputs since generally the cylinders are all operating atsubstantially the same air-fuel ratio. A more detailed description ofthis feedback control is provided in FIG. 2E and the related writtendescription. Note that in one particular example, according to thepresent invention, the air-fuel ratio of the cylinders combusting anair-fuel mixture when operating in the AIR/LEAN mode is controlled bycontrolling airflow entering the engine (see step 520). In this way, itis possible to control engine output by adjusting the fuel injection tothe combusting cylinders, while controlling the air-fuel ratio bychanging the air amounts provided to all of the cylinders.Alternatively, when engine 10 is not operating in the AIR/LEAN mode (seestep 522). The air-fuel ratio of all of the cylinders is controlled to adesired air-fuel ratio by changing the fuel injection amount, while thetorque output of the engine is adjusted by adjusting airflow to all ofthe cylinders.

Referring now to FIG. 6, a routine is described for determiningdegradation of exhaust gas oxygen sensors as well as controllingenablement of adaptive learning based on the exhaust gas oxygen sensors.

First, in step 610, the routine determines whether the engine isoperating in the AIR/LEAN mode. When the answer to step 610 is yes, theroutine continues to step 612 where a determination is made as towhether a sensor is exposed to a mixture of air and air plus combustedgases. When the answer to step 612 is no, the routine determines whetherthe sensor is exposed to pure air in step 614. When the answer to step614 is yes, the routine performs diagnostics of the sensor according tothe third method of the present invention (described later herein) anddisables adaptive learning (see FIG. 7). In other words, when a sensoris exposed only to a cylinder group that inducts air with substantiallyno injected fuel, then sensor diagnostics according to the third methodof the present invention are used, and adaptive learning of fueling andairflow errors is disabled.

Alternatively, when the answer to step 612 is yes, the routine continuesto step 618. In step 618, the routine performs diagnostics and learningaccording to the first method of the present invention described laterherein.

When the answer to step 614 is no, the routine continues to step 620 andperforms diagnostics and adaptive learning according to the secondmethod of the present invention (see FIG. 8).

When the answer to step 610 is no, the routine determines in step 622whether the engine is operating substantially near stoichiometry. Whenthe answer to step 622 is yes, the routine enables adaptive learningfrom the exhaust gas sensor in step 624. In other words, when allcylinders are combusting air and fuel, and the engine is operating nearstoichiometry, adaptive learning from the exhaust gas oxygen sensors isenabled. A more detailed description of adaptive learning is provided inFIG. 2F and the corresponding written description.

Then, in step 626, the routine enables stoichiometric diagnostics forthe sensors and catalyst.

Referring now to FIG. 7, the third adaptive/diagnostic method accordingto the present invention (see step 616 of FIG. 6) is described. First,in step 710, the routine determines whether the engine has been in theAIR/LEAN mode for a predetermined duration. This can be a predeterminedtime duration, a predetermined number of engine revolutions, or avariable duration based on engine and vehicle operating conditions, suchas vehicle speed and temperature. When the answer to step 710 is yes,the routine continues to step 712, where a determination is made as towhether the air fuel sensor indicates a lean air-fuel ratio. Forexample, the routine can determine whether the sensor indicates a leanvalue greater than a predetermined air-fuel ratio. When the answer tostep 712 is no, the routine increases count e by one in step 714. Then,in step 716, the routine determines whether count e is greater than afirst limit value (L1). When the answer to step 716 is yes, the routineindicates degradation of the sensor in step 718.

Thus, according to the present invention, when the sensor is coupledonly to a cylinder group inducting air with substantially no injectedfuel, the routine determines that the sensor has degraded when thesensor does not indicate a lean air-fuel ratio for a predeterminedinterval.

Referring now to FIG. 8, the second method of diagnostics and adaptivelearning according to the present invention (see step 620 of FIG. 6) isdescribed. First, in step 810, the routine determines whether theair-fuel sensor is functioning. This can be done in a variety ofmethods, such as, for example: comparing the measured air-fuel ratio toan expected air-fuel ratio value based on engine operating conditions.Then, in step 812, when the sensor is functioning properly, the routinecontinues to step 814. When the sensor has degraded, the routine movesfrom step 812 to step 816 and disables adaptive learning based on theair-fuel sensor reading.

Continuing with FIG. 8, when the answer to step 812 is yes, the routinedetermines whether fuel vapor is present in step 814. Again, if fuelvapor is present, the routine continues to step 816. Otherwise, theroutine continues to step 818 and learns an adaptive parameter toaccount for fuel injector aging, air meter aging, and various otherparameters as described in greater detail herein with particularreference to FIG. 2F. Adaptive learning can be in various forms, such asdescribed in U.S. Pat. No. 6,102,018 assigned to the assignee of thepresent invention and incorporated herein by reference in its entirety.

Referring now to FIG. 9, diagnostics and adaptive learning according tothe first method of the invention (see step 618 of FIG. 6) is described.First, in step 910, the routine determines whether the air-fuel sensoris functioning in a manner similar to step 810 in FIG. 8. Then, in step912, adaptive learning is disabled.

The method according to the present invention described hereinabove withparticular reference to FIGS. 6 through 9 describes diagnostics andadaptive learning for a particular exhaust gas oxygen, or air-fuelratio, sensor. The above routines can be repeated for each exhaust gassensor of the exhaust gas system.

Referring now to FIG. 10, the routine is described for estimatingcatalyst temperature depending on engine operating mode. First, in step1010, the routine determines whether the engine is operating in theAIR/LEAN mode. When the answer to step 1010 is no, the routine estimatescatalyst temperature using the conventional temperature estimatingroutines. For example, catalyst temperature is estimated based onoperating conditions such as engine coolant temperature, engine airflow,fuel injection amount, ignition timing, and various other parameters asdescribed in U.S. Pat. No. 5,303,168 for example. The entire contents ofU.S. Pat. No. 5,303,168 is incorporated herein by reference.

Alternatively, when the answer to step 1010 is no, the routine continuesto step 1014 where catalyst temperature is estimated taking into accountthe pure air effect based on the number of cylinders operating withoutinjected fuel. In other words, additional cooling from the airflowthrough cylinders without injected fuel can cause catalyst temperatureto decrease significantly. Alternatively, if the exhaust gases of thecombusting cylinders are rich, this excess oxygen from the cylindersoperating without injected fuel can cause a substantial increase inexhaust gas temperature. Thus, this potential increase or decrease tothe conventional catalyst temperature estimate is included.

Referring now to FIG. 11, a routine is described for controlling engineoperation in response to a determination of degradation of exhaust gassensors described above herein with particular reference to FIGS. 6through 9. In particular, in step 1110, the routine determines whetherany air-fuel sensors have been degraded. As described above, this can bedetermined by comparing the sensor reading to an expected value for thesensor reading. Next, when the answer to step 1110 is yes, the routinedetermines in step 1112 if the degraded sensor is used for enginecontrol during the AIR/LEAN mode of operation. If the answer to step1112 is yes, the routine disables the AIR/LEAN operation.

In other words, if a sensor has degraded that is used for engine controlduring the AIR/LEAN operating mode, then the AIR/LEAN operating mode isdisabled. Alternatively, if the sensor is not used in such an operatingmode, then the AIR/LEAN mode can be enabled and carried out despite thedegraded sensor.

Referring now to FIG. 12, a routine is described for controllingdisabling of the AIR/LEAN mode. First, in step 1201, the routinedetermines whether the engine is currently operating in the AIR/LEANmode. When the answer to step 1201 is yes, the routine continues to step1202 where it determines whether there is a request for anotheroperating mode. This request for another operating mode can take variousforms, such as, for example: the request for fuel vapor purging, therequest for operating rich to release and reduce NO_(X) trapped in theemission control device, the request for increasing brake booster vacuumby increasing manifold vacuum, a request for temperature management toeither increase a desired device temperature or decrease a desireddevice temperature, a request to perform diagnostic testing of variouscomponents such as sensors or the emission control device, a request toend lean operation, a request resulting from a determination that anengine or vehicle component has degraded, a request for adaptivelearning, or a request resulting from a control actuator reaching alimit value. When the answer to step 1202 is yes, the routine continuesto step 1203 where the AIR/LEAN mode is disabled.

Note that the request for fuel vapor purging can be based on variousconditions, such as the time since the last fuel vapor purge, ambientoperating conditions such as temperature, engine temperature, fueltemperature, or others.

As described above, if catalyst temperature falls too low (i.e., lessthan preselected value), operating some cylinders with substantially noinjected fuel can be disabled, and operating switched to firing allcylinders to generate more heat. However, other actions can also betaken to increase catalyst temperature. For example: ignition timing ofthe firing cylinders can be retarded, or, some fuel can be injected intothe non-combusting cylinders. In the latter case, the injected fuel canpass through (i.e., not ignited) and then react with excess oxygen inthe exhaust system and thereby generate heat.

Referring now to FIG. 13A, a routine is described for rapid heating ofthe emission control device. As described above herein, the emissioncontrol device can be of various types, such as, for example: athree-way catalyst, a NO_(x) catalyst, or various others. In step 1310,the routine determines whether the crank flag (crkflg) is set equal tozero. The crank flag indicates when the engine is being turned by theengine starter, rather than running under its own power. When it is setto one, this indicates that the engine is no longer in the crank mode.There are various methods known to determine when the engine hasfinished cranking such as, for example: when sequential fuel injectionto all of the engine cylinders has begun, or when the starter is nolonger engaged, or various other methods. Another alternative, ratherthan using an indication of engine cranking would be to use a flagindicating when the engine has begun synchronous fuel injection to allof the cylinders (sync_flg). In other words, when an engine starts, allof the cylinders are fired since engine position is not known. However,once the engine has reached a certain speed and after a predeterminedamount of rotation, the engine control system can determine whichcylinder is firing. At this point, the engine changes the sync_flg toindicate such a determination. Also note that during enginecranking/starting, the engine is operated substantially nearstoichiometry with all cylinders having substantially the same ignitiontiming (for example, MBT timing, or slightly retarded ignition timing).

When the answer to step 1310 is yes, the routine continues to step 1312where a determination is made as to whether the catalyst temperature(cat_temp) is less than or equal to a light off temperature. Note thatin an alternative embodiment, a determination can be made as to whetherthe exhaust temperature is less than a predetermined value, or whethervarious temperatures along the exhaust path or in different catalysthave reached predetermined temperatures. When the answer to step 1312 isno, this indicates that additional heating is not called for and theroutine continues to step 1314. In step 1314, the ignition timing of thefirst and second groups (spk_grp_1, spk_grp_2) set equal to base sparkvalues (base_spk), which is determined based on current operatingconditions. Also, the power heat flag (ph_enable) is set to zero. Notethat various other conditions can be considered for disabling the powerheat mode (i.e., disabling the split ignition timing). For example, ifthere is insufficient manifold vacuum, or if there is insufficient brakebooster pressure, or if fuel vapor purging is required, or if purging ofan emission control device such as a Nox trap is required. Similarly,when operating in the power heat mode, any of the above conditions willresult in leaving the power heat mode and operating all cylinders atsubstantially the same ignition timing. If one of these conditionsoccurs during the power heat mode, the transition routine describedbelow herein can be called.

Alternatively, when the answer to step 1312 is yes, this indicates thatadditional heating should be provided to the exhaust system and theroutine continues to step 1316. In step 1316, the routine sets theignition timing of the first and second cylinder groups to differingvalues. In particular, the ignition timing for the first group(spk_grp_1) is set equal to a maximum torque, or best, timing (MBT_spk),or to an amount of ignition retard that still provides good combustionfor powering and controlling the engine. Further, the ignition timingfor the second group (spk_grp_2) is set equal to a significantlyretarded valued, for example −29°. Note that various other values can beused in place of the 29° value depending on engine configuration, engineoperating conditions, and various other factors. Also, the power heatflag (ph_enable) is set to zero. Also, the amount of ignition timingretard for the second group (spk_grp_2) used can vary based on engineoperating parameters, such as air-fuel ratio, engine load, and enginecoolant temperature, or catalyst temperature (i.e., as catalysttemperature rises, less retard in the first and/or second groups, may bedesired). Further, the stability limit value can also be a function ofthese parameters.

Also note, as described above, that the first cylinder group ignitiontiming does not necessarily have to be set to maximum torque ignitiontiming. Rather, it can be set to a less retarded value than the secondcylinder group, if such conditions provide acceptable engine torquecontrol and acceptable vibration (see FIG. 13B). That is it can be setto the combustion stability spark limit (e.g., −10). In this way, thecylinders on the first group operate at a higher load than theyotherwise would if all of the cylinders were producing equal engineoutput. In other words, to maintain a certain engine output (forexample, engine speed, engine torque, etc.) with some cylindersproducing more engine output than others, the cylinders operating at thehigher engine output produce more engine output than they otherwisewould if all cylinders were producing substantially equal engine output.As an example, if there is a four cylinder engine and all cylinders areproducing a unitless output of 1, then the total engine output is 4.Alternatively, to maintain the same engine output of 4 with somecylinders operating at a higher engine output than others, then, forexample, two cylinders would have an output of 1.5, while the other twocylinders would have an output of 0.5, again for a total engine outputof 4. Thus, by operating some cylinders at a more retarded ignitiontiming than others, it is possible to place some of the cylinders into ahigher engine load condition. This allows the cylinders operating at thehigher load to tolerate additional ignition timing retard (or additionalenleanment). Thus, in these above examples, the cylinders operating witha unitless engine output of 1.5 could tolerate significantly moreignition timing retard than if all of the cylinders were operating at anengine output of 1. In this way, additional heat is provided to theengine exhaust to heat the emission control device.

An advantage to the above aspect of the present invention is that moreheat can be created by operating some of the cylinders at a higherengine load with significantly more ignition timing retard than ifoperating all of the cylinders at substantially the same ignition timingretard. Further, by selecting the cylinder groups that operate at thehigher load, and the lower load, it is possible to minimize enginevibration. Thus, the above routine starts the engine by firing cylindersfrom both cylinder groups. Then, the ignition timing of the cylindergroups is adjusted differently to provide rapid heating, while at thesame time providing good combustion and control.

Also note that the above operation provides heat to both the first andsecond cylinder groups since the cylinder group operating at a higherload has more heat flux to the catalyst, while the cylinder groupoperating with more retard operates at a high temperature. Also, whenoperating with a system of the configuration shown in FIG. 2C (forexample a V-8 engine), the two banks are substantially equally heatedsince each catalyst is receiving gasses from both the first and secondcylinder groups.

However, when using such an approach with a V-10 engine (for examplewith a system of the form of FIG. 2D), then the cylinder groups provideexhaust only to different banks of catalyst. As such, one bank may heatto a different temperature than the other. In this case, the aboveroutine is modified so periodically (for example, after a predeterminedtime period, or number of engine revolutions, etc.) the cylinder groupoperation is switched. In other words, if the routine starts with thefirst group operating with more retard than the second group, then aftersaid duration, the second group is operated with more retard than thefirst, and so on. In this way, even heating of the exhaust system isachieved.

When operating as described with regard to FIG. 13A, the engine operatessubstantially at, or lean of, stoichiometry. However, as describedbelow, with particular reference to FIGS. 13E-G, the air-fuel ratio ofthe cylinder groups can be adjusted to differing values as well.

Also note that all of the cylinders in the first cylinder group do notnecessarily operate at exactly the same ignition timing. Rather, therecan be small variations (for example, several degrees) to account forcylinder to cylinder variability. This is also true for all of thecylinders in the second cylinder group. Further, in general, there canbe more than two cylinder groups, and the cylinder groups can have onlyone cylinder. However, in one specific example of a V8, configured as inFIG. 2C, there are 2 groups, with four cylinders each. Further, thecylinder groups can be two or more.

Also note that, as described above, during operation according to FIG.13A, the engine cylinder air-fuel ratios can be set at different levels.In one particular example, all the cylinders are operated substantiallyat stoichiometry. In another example, all the cylinders are operatedslightly lean of stoichiometry. In still another example, the cylinderswith more ignition timing retard are operated slightly lean ofstoichiometry, and the cylinders with less ignition timing retard areoperated slightly rich of stoichiometry. Further, in this example, theoverall mixture air-fuel ratio is set to be slightly lean ofstoichiometry. In other words, the lean cylinders with the greaterignition timing retard are set lean enough such that there is moreexcess oxygen than excess rich gasses of the rich cylinder groupsoperating with less ignition timing retard. Operation according to thisalternate embodiment is described in more detail below, with particularreference to FIGS. 13E, 13F, 13G, and others.

In an alternative embodiment of the present invention, two differentcatalyst heating modes are provided. In the first mode, the engineoperates with some cylinders having more ignition timing retard thanothers. As described above, this allows the cylinders to operate atsubstantially higher load (for example, up to 70% air charge), since thecylinders with more retard are producing little torque. Thus, thecylinders with less retard than others can actually tolerate moreignition timing retard than if all cylinders were operating withsubstantially the same ignition timing retard while providing stablecombustion. Then, the remaining cylinders produce large amounts of heat,and the unstable combustion has minimal NVH (Noise, Vibration,Harshness) impacts since very little torque is being produced in thosecylinders. In this first mode, the air-fuel ratio of the cylinders canbe set slightly lean of stoichiometry, or other values as describedabove.

In a second mode, the engine operates with all of the cylinders havingsubstantially the same ignition timing, which is retarded to near thecombustion stability limit. While this provides less heat, it providesmore fuel economy. Further, the engine cylinders are operated nearstoichiometry, or slightly lean of stoichiometry. In this way, afterengine start-up, maximum heat is provided to the catalyst by operatingthe engine in the first mode until, for example, a certain time elapses,or a certain temperature is reached. Then, the engine is transitioned(for example, as described below herein) to operating with all cylindershaving substantially the same ignition timing retard. Then, once thecatalyst has reached a higher temperature, or another certain time haspassed, the engine is transitioned to operating near optimal ignitiontiming.

Referring now to FIG. 13B, the routine is described for transitioning inand out of the power seat strategy of FIG. 13A. The routine of FIG. 13Bis called by step 1314 of FIG. 13A. In other words, the routine providesthe following operation: first, the engine is started by operating allof the cylinders to combust an air and fuel mixture; and second, oncethe engine cylinders are firing synchronously, or engine speed hasreached a predetermined threshold, (and while the catalyst temperatureis below a desired light-off temperature) the engine is transitioned tooperate with one group of cylinders severely retarded and a second groupof cylinders with only so much ignition timing retard as can betolerated while providing acceptable engine combustion and minimumengine vibration. As described above, the cylinder group with a moreretarded timing can be operated, for example, about 10 degrees moreretarded than the less retarded cylinder group. However, this is justone example, and the difference can be various amounts, such as 5degrees, 10 degrees, 15 degrees, 20 degrees, 30 degrees, etc.

Also note that in this embodiment, both cylinder groups are operatingsubstantially at stoichiometry, or slightly lean of stoichiometry. Alsonote that engagement/disengagement of the A/C compressor can be disabledduring these transitions.

Referring now specifically to FIG. 13B, in step 1320 a determination ismade as to whether the power heat mode has been requested via anaffirmative answer to step 1312. In other words, the routine checkswhether the flag (ph_enable_flg) is set to 1. When the answer to step1320 is yes, the routine continues to step 1322 where a first rampingtimer (ph_ramp_tmr1) is said equal to zero. Then, in step 1324, theroutine determines whether the first ramping timer is greater than afirst ramp limit (rmp_lim_1). When the answer to step 1324 is no, theroutine continues to step 1326 where various operations are performed.In particular, in step 1326, the routine increments the first rampingtimer; calculates temporary spark retard value (spark_ret_tmp) based onthe maximum stability ignition timing retard that can be tolerated(max_stable_ret) and the first ramping timer and the first ramping timelimit. Further, the routine calculates the ignition timing for the firstand second groups (spk_grp_1, spk_grp_2) based on the optimum ignitiontiming (MBT_spk) and the temporary spark value. Further, the routineramps the airflow to increase. Alternatively, when the answer to step1324 is yes, the routine continues directly to step 1328.

In step 1328, the routine sets the first and second cylinder groupignition timing as follows: the second cylinder group ignition timing isset to severe retard (for example −29°), and the first cylinder groupignition timing is jumped up by an amount (spk_add_tq) necessary tocounteract the decrease in engine torque caused by setting the secondcylinder group to the severely retarded value. Further, in step 1328,the second ramping timer is set equal to zero.

Next, in step 1330, the routine determines whether the second rampingtimer (Rmp_tmr_2) is greater than a limit time (Rmp_lim_2). When theanswer to step 1330 is no, the routine continues to step 1332. In step1332, the first cylinder group ignition timing is gradually decreasedbased on the ramping timer. Further, the second ramping timer isincremented and airflow is gradually increased. Alternatively, when theanswer to step 1330 is yes, the routine ends.

In this way, it is possible to transition from all cylinders operatingwith substantially equal ignition timing to operating with a first groupof cylinders severely retarded, and a second group of cylindersgenerating increased engine torque than if all cylinders were operatingat substantially full ignition timing. The routine of FIG. 13B can bemore fully understood by considering the graphs of FIG. 13C. The graphshows engine airflow, ignition timing for the two cylinder groups versustime. Ignition timing for cylinder group 1 and group 2 is shown in FIGS.13C(2) and 13C(3), respectively. Before time t0, the engine is stopped.At time t0, the engine is cranked/started. Then, at time t1, the enginehas reached a predetermined engine speed and all cylinders are beingfired synchronously. At time t1, airflow is gradually increased whilethe ignition timing of both cylinder groups is retarded from optimum(nbt) timing. Then, at time t2, both cylinders have been retarded to thecombustion stability limit (for example 0°). Up to this point, allcylinders are firing and producing substantially similar engine output.At time t2, the ignition timing on the second cylinder group is jumpedto a severely retarded value (for example −29°) as shown in FIG. 13C(3).Similarly, at this time, the ignition timing on the first cylinder groupis jumped back towards optimum ignition timing as shown in FIG. 13C(2).In particular, the amount of this jump on the first cylinder group isbased on the amount of torque increase needed to cancel the torquedecrease caused by the retard on the second cylinder group. Then, attime t3, the ignition timing on the first cylinder group is graduallyramped back towards the stability limit, while the airflow is againgradually increased to maintain engine torque until time t4. Thus,according to the present invention, it is possible to adjust airflow(via the throttle or other parameters such as variable cam timing) whileadjusting ignition timing as described above to transition the engine tooperating with some cylinders severely retarded and others retarded onlyto a predetermined threshold, while maintaining engine torquesubstantially constant. The remainder of FIG. 13C will be describe belowherein after description of the reverse transitions in FIG. 13D.

Referring now to FIG. 13D, a routine is described for transitioning fromoperating with some cylinder groups having more retarded ignition timingthan others to operating with other cylinders as substantially the sameignition timing. In particular, the routine of FIG. 13D is called bystep 1314 of FIG. 13A. First, in step 1340, the routine determineswhether the power heat flag is set to zero. When the answer to step 1340is yes, the routine continues to step 1342. In step 1342, the routinesets the second ramp timer to zero. Then, in step 1344, the routinedetermines whether the second ramp timer is greater than a second ramplimit. When the answer to step 1344 is no, the routine continues to step1346. In step 1346, the routine increments the second ramp timer andsets the ignition timing for the first cylinder group to ramp based onthe second ramp timer and the first ramp limit, as well as the ignitiontiming adjustment based on the change in torque. Further, the routinedecreases airflow. Next, in step 1350, the routine sets the first andsecond ignition timings as shown in the Figure. Further, the routinesets the first ramp timer to zero. In particular, the routine sets thefirst ignition timing to jump based on the additional torque, or clipsit to the stability limit. Next, in step 1352, the routine determineswhether the first ramp timer is greater than the first timer limit. Whenthe answer to step 1352 is no, the routine continues to step 1354. Instep 1354, the routine sets the first and second cylinder group ignitiontiming as described, as well as incrementing the first ramp timer andincreasing airflow.

Operation according to FIG. 13D can be more fully understood by againconsidering FIG. 13C. As described above, at time t4, the engine isoperating at a high airflow with the first cylinder group having anignition timing retard to the stability limit, while the second cylindergroup has an ignition timing that is severely retarded past thestability limit, thereby generating heat to the engine exhaust. At timet5, the routine decreases engine airflow while increasing the ignitiontiming on the first cylinder group towards optimum ignition timing untiltime t6. Then, at time t7, the routine jumps the ignition timing on thefirst cylinder group towards the stability limit, while at the same timejumping the ignition timing on the second cylinder group to thestability limit. Then, from time t7 to time t8, engine airflow isfurther decreased, while the ignition timing on both cylinder groups isramped towards optimal ignition timing. In this way, the routinetransitions to operating all of the cylinders at substantially the sameignition timing near the optimum ignition timing.

Referring now to FIG. 13E, a routine is described for transitioning theengine air fuel ratio after the engine has transitioned to operatingwith one group of cylinders having an ignition timing more retarded thananother group of cylinders. In particular, the routine describes how totransition to operate one group of cylinders with a slightly rich bias,and the other group of cylinders with a slightly lean bias. Further, thelean and rich bias values are selected such that the overall mixtureair-fuel ratio of gasses from the first and second cylinder groups isslightly lean of stoichiometry, for example, between 0.1 and 1. air-fuelratios. First, in step 1360, the routine determines whether the engineis currently operating in the power-heat mode (operating one cylindergroup with an ignition timing more retarded than another cylindergroup). When the answer to step 1360 is yes, the routine continues tostep 1361, where the air fuel ratio timer (ph_lam_tmr1) is set equal tozero. Then, the routine continues to step 1362, where a determination ismade as to whether the air-fuel ratio timer is greater than a firstlimit value (ph_lam_tim1). When the answer to step 1362 is no, theroutine continues to step 1363. In step 1363, the timer is incremented,and the first and second cylinder group desired air-fuel ratios(lambse_1, lambse2) are ramped to the desired values, while airflow isadjusted to maintain engine torque substantially constant. Inparticular, while the airflow ratios are ramped, the engine airflow isincreased. In particular, the torque ratio (tq_ratio) is calculatedusing function 623. Function 623 contains engine mapping data that givesa relationship between the engine torque ratio and the air-fuel ratio.Thus, from this function and the equations described in step 1363, it ispossible to calculate the desired airflow to maintain engine torquesubstantially constant while changing the combustion air-fuel ratios.Then, in step 1364, the timer is reset to zero.

Thus, as described in FIG. 13E above, the engine is transitioned fromoperating all of the cylinders at substantially the same air-fuel ratio(with one cylinder group operating at an ignition timing more retardedthan others) to operating first group of cylinders at a first ignitiontiming with a first air-fuel ratio slightly rich, and a second group ofcylinders operating a second ignition timing substantially more retardedthan the first ignition timing, and at a second air-fuel ratio slightlylean of stoichiometry. This operation can be more fully understood byconsidering the first portion of FIG. 13G. In particular, the FIG.13G(1) shows the spark transition described above herein with particularreference to FIG. 13B. FIG. 13G(2), shows an air-fuel ratio transitionaccording to FIG. 13E. Note that the desired airflow adjustment that ismade to compensate for the change in air-fuel ratio of the first andsecond cylinder groups may cause airflow to increase in some conditions,while causing air flow to decrease in other conditions. In other words,there may be conditions that require increasing engine airflow tomaintain substantially the same engine torque, while there may also beother conditions that require decreasing engine air flow to maintainengine torque substantially constant. FIG. 13G(3) will be described morefully below after a description of FIG. 13F.

Referring now to FIG. 13F, routine is described for transitioning out ofthe split air-fuel ratio operation. First, in step 1365, the routinedetermines whether the engine is operating in the power heat mode bychecking the flag (ph_running_flg). When the answer to step 1365 is yes,the routine continues to step 1366 where the second air fuel ratio timer(ph_lam_tmr2) is set to zero. Next, in step 1367, the routine determineswhether the timer is greater than a limit value (ph_lam_tim2). When theanswer to step 1367 is no, the routine continues to step 1368.

In step 1368, the timer is incremented, and the first and secondcylinder group desired air-fuel ratio is open (lambse_1, lambse_2) arecalculated to maintain engine torque substantially constant. Further,the desired air flow is calculated based on the torque ratio andfunction 623. Further, these desired air-fuel ratios are calculatedbased on the desired rich and lean bias values (rach_bias, lean_bias).As such, in a manner similar to step 1363, the air-fuel ratios areramped while the airflow is also gradually adjusted. Just as in step1363, the desired air-fuel ratio may increase or decrease depending onoperating conditions. Finally, in step 1369, the timer is reset to zero.

Operation according to FIG. 13F can be more fully understood bycontinuing the second half of the graph in FIG. 13G. Continuing thedescription of 13G from above, after the air fuel transition into thesplit air-fuel operating mode, the Figure shows transitioning out of thesplit air-fuel ratio mode, where the desired air-fuel ratios are rampedto a common value. Similarly, the airflow is adjusted to compensateengine torque.

Referring now to FIG. 13H, a routine is described for controlling engineidle speed during the power heat mode. In other words, after the engineis started by firing all the cylinders, and the engine is transitionedto operating with a first group of cylinders having more ignition timingretard than a second group of cylinders, FIG. 13H describes the controladjustments made to maintain engine idle speed during such operation.First, in step 1370, the routine determines whether the engine is in theidle speed control mode. When the answer to step 1370 is yes, theroutine continues to step 1371 where a determination is made as towhether the engine is operating in the power heat mode by checking aflag (ph_running_flg). When the answer to step 1371 is yes, the engineis operating with the first cylinder group having more ignition timingretard than a second cylinder group. When the answer to step 1371 isyes, the routine continues to step 1372 and calculates an engine speederror between a desired engine idle speed and a measured engine idlespeed. Then, in step 1373, the routine calculates an airflow adjustmentvalue based on the speed error, as well as an adjustment to the firstcylinder group ignition timing based on the speed error. In other words,the routine adjusts airflow to increase when engine speed falls belowthe desired value, and adjust airflow to decrease when engine speedrises above the desired value. Similarly, when engine speed falls belowthe desired value, the ignition timing of the first cylinder group(spk_grp_1) is advanced toward optimal emission timing. Further, whenengine speed rises above the desired value, the ignition timing of thefirst cylinder group is retarded away from optimal ignition timing.

When the answer to step 1371 is no, the routine continues to step 1374and calculates an engine idle speed error. Then, in step 1375, theroutine adjusts airflow based on the speed error, as well as both thefirst and second cylinder group ignition timing values based on thespeed error. In other words, when not in the power heat mode, the engineadjusts the ignition timing to all cylinders to maintain engine idlespeed.

Referring now to FIG. 13K, an alternate embodiment of the routinedescribed in FIG. 13H is described. Steps 1380, 1381, 1382, 1386, and1387 correspond to steps 1370, 1371, 1372, 1374, and 1375 of step 13H.However, in FIG. 13K, the routine has an additional check to determinewhether the control authority of ignition timing of the first cylindergroup has reached a limit value. In particular, in step 1384, theroutine determines whether the first ignition timing (spk_grp_1) isgreater than the optimal ignition timing (MBT-SPK). In other words, theroutine determines whether the ignition timing of the first cylindergroup has been advanced to the maximum ignition timing limit. When theanswer to step 1384 is yes, the routine continues to step 1385 and setsthe first cylinder group ignition timing to the optimal ignition timingand calculates an adjustment to the second cylinder group ignitiontiming based on a speed error.

In other words, if a large engine load is placed on the engine andadjustment of engine air flow and the first cylinder group ignitiontiming to the optimal ignition timing is insufficient to maintain thedesired engine idle speed, then additional torque is supplied from thesecond cylinder group by advancing the ignition timing towards theoptimal ignition timing. While this reduces the engine heat generated,it only happens for a short period of time to maintain engine idlespeed, and therefore, has only a minimal effect on catalyst temperature.Thus, according to the present invention, it is possible to quicklyproduce a very large increase in engine output since the engine hassignificant amount of ignition timing retard between the first andsecond cylinder groups.

Note that FIG. 13C shows operation where desired engine torque issubstantially constant. However, the routines of FIGS. 13A, B andothers, can be adjusted to compensate for a change in desired engineoutput by adjusting engine airflow to provide the desire engine output.That is, the airflow can have a second adjustment value to increase ordecrease engine airflow from the values shown to accommodate such arequest. In other words, during the very short time of the transition,the desired engine output can be maintained substantially constant ifdesired, or increased/decreased by further adjusting engine airflow fromthat shown.

Note that in the above described idle speed control operations, air/fuelor spark transitions may be smoothed by engaging or disengaging anengine load such as this AC compressor.

Referring now to FIG. 13I, several examples of operation of an engineare described to better illustrate operation according to the presentinvention and its corresponding advantages. These examples schematicallyrepresent engine operation with differing amounts of air, fuel, andignition timing. The examples illustrate schematically, one cylinder ofa first group of cylinders, and one cylinder of a second group ofcylinders. In Example 1, the first and second cylinder groups areoperating with substantially the same air flow, fuel injection, andignition timing. In particular, the first and second groups induct anair flow amount (a1), have injected fuel amount (f1), and have anignition timing (spk1). In particular, groups 1 and 2 in Example 1 areoperating with the air and fuel amounts in substantially stoichiometricproportion. In other words, the schematic diagram illustrates that theair amount and fuel amount are substantially the same. Also, the Example1 illustrates that the ignition timing (spk1) is retarded from optimaltiming (MBT). Operating in this way results in the first and secondcylinder groups producing an engine torque (T1).

Example 2 of FIG. 13I illustrates operation according to the presentinvention. In particular, the ignition timing of the second group(spk2′) is substantially more retarded than the ignition timing of thefirst cylinder group of Example 2 (spk2). Further, the air and fuelamounts (a2, f2) are greater than the air amounts in Example 1. As aresult of operation according to Example 2, the first cylinder groupproduces engine torque (T2), while the second cylinder group producesengine torque (T2′). In other words, the first cylinder group producesmore engine torque than when operating according to Example 1 sincethere is more air and fuel to combust. Also note that the first cylindergroup of Example 2 has more ignition retard from optimal timing than theignition timing of group 1 of Example 1. Also, note that the enginetorque from the second cylinder group (T2′) is less than the enginetorque produced by the first and second cylinder group of Example 1, dueto the severe ignition timing retard from optimal timing. The combinedengine torque from the first and second cylinder groups of Example 2 canbe roughly the same as the combined engine torque in the first andsecond cylinder groups of Example 1. However, significantly more exhaustheat is generated in Example 2 due to the large ignition timing retardof the second group, and the ignition timing retard of the first groupoperating at a higher engine load.

Referring now to Example 3 of FIG. 13I, operation according to anotherembodiment of the present invention is described. In Example 3, anaddition to adjustments of ignition timing, the first cylinder group isoperated slightly rich, and the second cylinder group is operatedslightly lean. Also note that these cylinder groups can be operated atvarious rich and lean levels. Operation according to the third exampleproduces additional heat since the exhaust temperature is high enoughsuch that the excess fuel of the first group reacts with the excessoxygen from the second group.

Referring now to FIG. 13J, a graph is shown illustrating engine airflowversus throttle position. According to operation of the presentinvention, in one particular example an electronically controlledthrottle is coupled to the engine (instead of, for example, a mechanicalthrottle and an idle air pass valve). FIG. 13J shows that at lowthrottle positions, a change in throttle position produces a largechange in air flow, while at large throttle positions, a change inthrottle position produces a relatively smaller change in air flow. Asdescribed above herein, operation according to the present invention(for example, operating some cylinders at a more retarded ignitiontiming than others, or operating some cylinders without fuel injection)causes the engine cylinders to operate at a higher load. In other words,the engine operates at a higher airflow and larger throttle position.Thus, since the slope of airflow to throttle position is lower in thisoperating mode, the controllability of airflow, and torque, is therebyimproved. In other words, considering the example of idle speed controlvia adjustments of throttle, engine idle speed is better maintained atthe desired level. For example, at throttle position (tp1) the sloperelating air flow and throttle position is s1. At throttle position(tp2), the slope is s2, which is less than slope s1. Thus, if the enginewere operating with all cylinders at substantially the same ignitiontiming, the throttle position may be operating about throttle position(tp1). However, if the engine is operating at a higher load (since somecylinders are operating with more ignition timing retard than others),then the engine can operate about throttle position (tp2). As such,better idle speed control can be achieved.

As described above, engine idle speed control is achieved by adjustingignition timing during the power heat mode. Note that various alternateembodiments are possible. For example, a torque based engine idle speedcontrol approach could be used. In this approach, from the desiredengine speed and engine speed error, a desired engine output (torque) iscalculated. Then, based on this desired engine torque, an airflowadjustment and ignition timing adjustment value can be calculated.

Referring now to FIG. 14, an alternate embodiment is described forquickly heating the exhaust system. Note that the routine of FIG. 14 isapplicable to various system configurations, such as systems whereexhaust gasses from the cylinder groups mix at some point before theyenter the catalyst to be heated.

First, in step 1410, the routine determines whether the crank flag isset to zero. Note that when the crank flag is set to zero, the engine isnot in the engine start/crank mode. When the answer to step 1410 is yes,routine continues to step 1412. In step 1412, the routine determineswhether the catalyst temperature (cat_temp) is above a first temperature(temp1) and below a second temperature (temp2). Various temperaturevalues can be used for temp1 and temp2, such as, for example: settingtemp1 to the minimum temperature that can support a catalytic reactionbetween rich gasses and oxygen, setting temp2 to a desired operatingtemperature. When the answer to step 1412 is no, the routine does notadjust the engine ignition timing (spark retard).

Alternatively, when the answer to step 1412 is yes, the routinecontinues to step 1414. In step 1414, the routine adjusts engineoperation to operate with one cylinder group receiving injecting fueland inducting air, and the second group inducting air with substantiallyno injector fuel. More specifically, if the engine was started with allcylinders (i.e., all cylinders are currently firing) then the enginetransitions to operating with only some cylinders firing, such asdescribed above herein with particular reference to FIG. 3D(2), forexample. Also, once the engine has been transitioned, the cylinders thatare combusting air and fuel are operated at an air-fuel ratio which isrich of stoichiometry. However, the firing cylinder air-fuel ratio isnot set so rich such that the mixture of the combusted gasses with theair from the non-combusting cylinders is substantially greater than nearstoichiometry. In other words, the mixture air-fuel ratio is maintainedwithin a limit (above/below) near the stoichiometric value. Next, instep 1416, the routine sets the ignition timing, for the firingcylinders, to a limited value. In other words, the ignition timing forthe firing cylinders are set to, for example, the maximum ignitiontiming retard that can be tolerated at the higher engine load, whileproducing acceptable engine control and engine vibration.

In this way, the rich combustion gasses from the firing cylinders canmix with and react with the excess oxygen in the cylinders withoutinjected fuel to created exothermic or catalytic heat. Further, heat canbe provided from the firing cylinders operating at a higher load thanthey otherwise would if all cylinders were firing. By operating at thishigher load, significant ignition timing retard can be tolerated whilemaintaining acceptable engine idle speed control and acceptablevibration. Further, since the engine is operating at a higher load, theengine pumping work is reduced.

Also note that once the desired catalyst temperature, or exhausttemperature, has been reached, the engine can transition back tooperating with all cylinders firing, if desired. However, when theengine is coupled to an emission control device that can retain NOx whenoperating lean, it may be desirable to stay operating in the mode withsome cylinders firing and other cylinders operating with substantiallyno injected fuel. However, once the desired catalyst temperature isreached, the mixture air-fuel ratio can be said substantially lean ofstoichiometry. In other words, the firing cylinders can be operated witha lean air-fuel ratio and the ignition timing set to maximum torquetiming, while the other cylinders operate with substantially no injectedfuel.

Referring now to FIG. 15, another alternate embodiment of the presentinvention is described for heating the exhaust system. In thisparticular example, the routine operates the engine to heat the emissioncontrol device to remove sulfur (SO_(X)) that has contaminated theemission control device. In step 1510, the routine determines whether adesulfurization period has been enabled. For example, a desulfurizationperiod is enabled after a predetermined amount of fuel is consumed. Whenthe answer to step 1510 is yes, the routine continues to step 1512. Instep 1512, the routine transitions from operating with all cylindersfiring to operating with some cylinders firing and other cylindersoperating with substantially no injected fuel. Further, the firingcylinders are operated at a significantly rich air-fuel ratio, such as,for example 0.65. Generally, this rich air-fuel ratio is selected asrich as possible, but not so rich as to cause soot formation. However,less rich values can be selected. Next, in step 1514, the routinecalculates a mixture air-fuel ratio error in the exhaust systemtailpipe. In particular, a tailpipe air-fuel ratio error (TP_AF_err) iscalculated based on the difference between an actual tailpipe air-fuelratio (TP-AF) minus a desired, or set-point, air-fuel ratio (set_pt).Note that the actual air-fuel ratio and tailpipe can be determined froman exhaust gas oxygen sensor positioned in the tailpipe, or estimatedbased on engine operating conditions, or estimated based on air-fuelratios measured in the engine exhaust.

Next, in step 1516, the routine determines whether the tailpipe air fuelair is greater than zero. When the answer to step 1516 is yes, (i.e.there is a lean error), the routine continues to step 1518. In step1518, the airflow into the group operating with substantially noinjected fuel is reduced. Alternatively, when the answer to step 1516 isno, the routine continues to step 1520 where the airflow into the groupoperating with substantially no injected fuel is increased. Note thatthe airflow into the group operating with substantially no injected fuelcan be adjusted in a variety of ways. For example, it can be adjusted bychanging the position of the intake throttle. However, this also changesthe airflow entering the cylinder's combusting air and fuel and thusother actions can be taken to minimize any affect on engine outputtorque. Alternatively, the airflow can be adjusted by changing the camtiming/opening duration of the valves coupled to the group operatingsystem with substantially no injected fuel. This will change the airflowentering the cylinders, with a smaller affect on the airflow enteringthe combusted cylinders. Next, in step 1522, a determination is made asto whether the catalyst temperature has reached the desulfurizationtemperature (desox_temp). In this particular example, the routinedetermines whether the downstream catalyst temperature (for examplecatalyst 224) has reached a predetermined temperature. Further, in thisparticular example, the catalyst temperature (ntrap_temp) is estimatedbased on engine operating conditions. Also note, that in this particularexample, the downstream catalyst is particularly susceptible to sulfurcontamination, and thus it is desired to remove sulfur in thisdownstream catalyst. However, sulfur could be contaminating upstreamemission control devices, and the present invention can be easilyaltered to generate heat until the upstream catalyst temperature hasreached its desulfurization temperature.

When the answer to step 1522 is yes, the routine reduces the air-fuelratio in the cylinder and the combusting cylinders. Alternatively, whenthe answer to step 1522 is no, the routine retards ignition timing andincreases the overall airflow to generate more heat.

In this way, heat is generated from the mixture of the combusted richgas mixture and the oxygen in the airflow from the cylinders operatingwith substantially known injected fuel. The air-fuel ratio of themixture is adjusted by changing the airflow through the engine. Further,additional heat can be provided by retarding the ignition timing of thecombusting cylinders, thereby increasing the overall airflow to maintainthe engine output.

As a general summary, the above description describes a system thatexploits several different phenomena. First, as engine load increases,the lean combustion limit also increases (or the engine is simply ableto operate lean where it otherwise would not be). In other words, as theengine operates at higher loads, it can tolerate a lean(er) air-fuelratio and still provide proper combustion stability. Second, as engineload increases, the ignition timing stability limit also increases. Inother words, as the engine operates at higher loads, it can toleratemore ignition timing retard and still provide proper combustionstability. Thus, as the present invention provides various methods forincreasing engine load of operating cylinders, it allows for the higherlean air-fuel ratio or a more retarded ignition timing, for the sameengine output while still providing stable engine combustion for somecylinders. Thus, as described above, both the ignition timing retardstability limit, and the lean combustion stability limit are a functionof engine load.

While the invention has been described in detail, those familiar withthe art to which this invention relates will recognize variousalternative designs and embodiments for practicing the invention asdefined by the following claims.

What is claimed:
 1. A method for controlling an engine having first andsecond groups of combustion chambers and an engine exhaust, comprising:operating the engine in a first operating mode where air andsubstantially no injected fuel are provided to the first combustionchamber group, and both air and injected fuel are provided to the secondcombustion chamber group at a lean air-fuel ratio, and injected fuel isadjusted to the second combustion chamber group; and operating theengine in a second mode where both air and injected fuel are provided toboth the first and second combustion chamber groups, and at least one ofsaid air and said injected fuel provided to both said first and secondcombustion groups is adjusted to control air-fuel ratio of the first andsecond groups.
 2. The method recited in claim 1 wherein in said firstmode, injected fuel is adjusted to the second combustion chamber basedon a desired engine output.
 3. The method recited in claim 1 wherein insaid second mode, said at least one of said air and said injected fuelprovided to both said first and second combustion groups is adjusted tocontrol air-fuel ratio of the first and second groups based on a desiredair-fuel ratio and a sensor output of a sensor coupled to the engineexhaust.
 4. A method for controlling an engine having first and secondgroups of combustion chambers, comprising: operating the engine in afirst mode where fuel and air are provided to the first combustionchamber group, air and substantially no injected fuel are provided tothe second combustion chamber group, and said fuel provided to saidfirst combustion chamber group is adjusted to adjust engine output, andairflow through the engine is adjusted to control an air-fuel ratio ofthe first combustion chamber group; and operating the engine in a secondmode other than said first mode, where a parameter other than fuelinjected only to said first combustion chamber group is adjusted tocontrol air-fuel ratio of the engine.
 5. The method recited in claim 4wherein when operating in the first mode, said air-fuel ratio of thefirst combustion chamber group is controlled to a value lean ofstoichiometry.
 6. The method recited in claim 4 wherein when operatingin the second mode, said parameter is fuel injected to all cylinders insaid first and second groups.
 7. The method recited in claim 4 whereinwhen operating in the second mode, said parameter is airflow.
 8. Themethod recited in claim 4 wherein said engine output is engine torque.9. A method for controlling an engine, comprising: reading a sensoroutput of a sensor exposed to a mixture of air and substantially noinjected fuel in a first stream from the engine and a combusted leanmixture of air and fuel in a second stream from the engine; adjusting ameasured air-fuel ratio of said combusted lean air and fuel mixture toaccount for said air in said first stream; and treating said combustedair and fuel in a catalyst coupled to the engine that retains oxidantsincluding NO_(X) when operating lean and reduces said retained oxidantswhen operating rich.
 10. The method recited in claim 9 wherein saidsensor is an exhaust gas oxygen sensor.
 11. The method recited in claim9 wherein said catalyst comprises platinum and barium.
 12. A method forcontrolling an engine having first and second groups of combustionchambers, comprising: operating the engine in a first mode where air andinjected fuel are provided to the first combustion chamber group at alean air-fuel ratio, air and substantially no injected fuel are providedto the second combustion chamber group, adjusting one of said air andsaid injected fuel provided to said first combustion chamber group toadjust engine output, and adjusting the other of said air and saidinjected fuel to control an air-fuel ratio of the first combustionchamber group; and operating the engine in a second mode other than saidfirst mode and adjusting a parameter other than said one of said air andinjected fuel provided to said first combustion chamber group to controlair-fuel ratio of the engine.
 13. A method for controlling an enginehaving first and second groups of combustion chambers coupled to anemission control device, comprising: operating the engine in a modewhere air and injected fuel are provided to the first combustion chambergroup, air and substantially no injected fuel are provided to the secondcombustion chamber group; adjusting one of said air and said injectedfuel provided to said first combustion chamber group to adjust air-fuelratio of said first group to change between lean and rich; measuringtemperature of the emission control device; and calculating air-fuelratio of the first cylinder group based on said temperature.